A blue hole is a flooded sea cave with a hole that opens up at the land's surface. These cave systems form in carbonate rock, often on islands. Some blue holes have very special rock formations and water chemistry. Far below sea level, they contain some of the harshest environments on Earth, with no oxygen and no light. Yet these areas are filled with life forms that have adapted to the extreme conditions. Information gathered from these blue holes is helping scientists to increase their understanding of biology, archaeology and geology. But exploring these blue hole environments brings danger as well as discovery.  Blue holes get their name from the color1 some have when seen from the air. The color1 is usually a reflection of the sky on the water. But not all of these cave systems have blue surfaces. Some contain dark or muddy water. Blue holes are the result of erosion in which water breaks down rock. Rain falling thousands of years ago contained chemicals which slowly wore away at the limestone landmass. These holes were later filled with sea water as the sea level changed. The rising and falling of sea levels and the mixture of salt and fresh water further wore away at these cave formations. Blue holes are vertical caves. But they can also have horizontal cave formations that may be hundreds of meters long.  These flooded cave systems can be found in the ocean, or they can be found inland. Ocean caves are affected by tides, so they always have water movement. But blue holes on land are very still. They have several layers of water, chemicals, and bacteria. The top layer of fresh water comes from rainfall. This layer acts like a cap on top of the layered mixture, and keeps out oxygen from the atmosphere. The fresh water floats on a denser layer of saltwater. Underneath this is a layer of poisonous hydrogen sulfide, produced by bacteria living in the water. Underneath this layer is anoxic seawater -- water that does not contain any oxygen.  Underwater caves are probably one of the least explored ecosystems on the planet. One reason blue holes have not been fully explored is that they can be extremely dangerous. There are many safety rules that divers must follow to help ensure their survival. First, divers must have training and experience to swim in these caves. Those who explore a cave for the first time must establish a thin rope called a guideline. This line helps them to safely enter and bring more than one set of breathing equipment in case one device fails. Meanwhile, they must pay careful attention to their air supply.  The rule is to use a third of their air to enter the cave, a third to exit, and a third for emergencies.          For most explorers, though, the possibility of discovery in these cave environments makes the experience worth the risk. You can jump into what looks like an insignificant little hole in the ground, and come out with information that's of value to many different disciplines, Scientists are interested in these caves because oxygen-free conditions there are similar to those on Earth long ago, before oxygen existed on our planet., as well as similar to space. According to the Passage, blue holes  _  .
are now a popular choice for tourists are flooded caves found mostly on islands have appeared as rocks pile up year by year can give off bright blue light deep in the caves
Question: A blue hole is a flooded sea cave with a hole that opens up at the land's surface. These cave systems form in carbonate rock, often on islands. Some blue holes have very special rock formations and water chemistry. Far below sea level, they contain some of the harshest environments on Earth, with no oxygen and no light. Yet these areas are filled with life forms that have adapted to the extreme conditions. Information gathered from these blue holes is helping scientists to increase their understanding of biology, archaeology and geology. But exploring these blue hole environments brings danger as well as discovery.  Blue holes get their name from the color1 some have when seen from the air. The color1 is usually a reflection of the sky on the water. But not all of these cave systems have blue surfaces. Some contain dark or muddy water. Blue holes are the result of erosion in which water breaks down rock. Rain falling thousands of years ago contained chemicals which slowly wore away at the limestone landmass. These holes were later filled with sea water as the sea level changed. The rising and falling of sea levels and the mixture of salt and fresh water further wore away at these cave formations. Blue holes are vertical caves. But they can also have horizontal cave formations that may be hundreds of meters long.  These flooded cave systems can be found in the ocean, or they can be found inland. Ocean caves are affected by tides, so they always have water movement. But blue holes on land are very still. They have several layers of water, chemicals, and bacteria. The top layer of fresh water comes from rainfall. This layer acts like a cap on top of the layered mixture, and keeps out oxygen from the atmosphere. The fresh water floats on a denser layer of saltwater. Underneath this is a layer of poisonous hydrogen sulfide, produced by bacteria living in the water. Underneath this layer is anoxic seawater -- water that does not contain any oxygen.  Underwater caves are probably one of the least explored ecosystems on the planet. One reason blue holes have not been fully explored is that they can be extremely dangerous. There are many safety rules that divers must follow to help ensure their survival. First, divers must have training and experience to swim in these caves. Those who explore a cave for the first time must establish a thin rope called a guideline. This line helps them to safely enter and bring more than one set of breathing equipment in case one device fails. Meanwhile, they must pay careful attention to their air supply.  The rule is to use a third of their air to enter the cave, a third to exit, and a third for emergencies.          For most explorers, though, the possibility of discovery in these cave environments makes the experience worth the risk. You can jump into what looks like an insignificant little hole in the ground, and come out with information that's of value to many different disciplines, Scientists are interested in these caves because oxygen-free conditions there are similar to those on Earth long ago, before oxygen existed on our planet., as well as similar to space. According to the Passage, blue holes  _  .
A. are now a popular choice for tourists
B. are flooded caves found mostly on islands
C. have appeared as rocks pile up year by year
D. can give off bright blue light deep in the caves
Answer: