"I stepped out and was _ ," local journalist Bahram Baloch told the BBC. "I could see this gray, dome-shaped   body in the distance, like a giant whale swimming near the surface. Hundreds of people had gathered to watch it in disbelief." This island has become a global curiosity. How was it formed in just a few minutes? As you might already know, many islands in the sea are formed by volcanoes. There are numerous volcanoes under the sea. As the hot lava erupts and cools down, it piles up and forms the shape of a mountain. When the mountain "grows" to higher than the sea level, the part that's above the water is what we call an "island". This is how the island was formed after the Pakistan earthquake, except that instead of a regular volcano, it was a "mud volcano" that brought about this island. Lava is not the only thing that's locked under the Earth's crust -- there is also gas. When an earthquake happens and breaks part of the crust, the gas is released at an extremely high speed, pushing mud up to the surface, according to National Geographic. But only earthquakes that are extremely powerful can cause mud volcanoes to push up enough mud to produce islands -- and this 7.7-magnitude earthquake in Pakistan was strong enough. The island is about 20 meters high, up to about 90 meters wide and 30 meters long, nearly the size of a soccer field. In fact, mud volcano islands aren't new. This is the fourth island of this kind in the region since 1945. But those islands usually didn't last long. "It will probably be gone within a couple of months," said Bill Barnhart, a researcher with the US Geological Survey. After all, "it's just a big pile of mud that was on the seafloor that got pushed up". According to the article, islands formed by mud volcanoes   _  .
are quite difficult to locate usually disappear after a short period of time are formed by hot lava from volcanoes used to be part of the Earth's crust
Question: "I stepped out and was _ ," local journalist Bahram Baloch told the BBC. "I could see this gray, dome-shaped   body in the distance, like a giant whale swimming near the surface. Hundreds of people had gathered to watch it in disbelief." This island has become a global curiosity. How was it formed in just a few minutes? As you might already know, many islands in the sea are formed by volcanoes. There are numerous volcanoes under the sea. As the hot lava erupts and cools down, it piles up and forms the shape of a mountain. When the mountain "grows" to higher than the sea level, the part that's above the water is what we call an "island". This is how the island was formed after the Pakistan earthquake, except that instead of a regular volcano, it was a "mud volcano" that brought about this island. Lava is not the only thing that's locked under the Earth's crust -- there is also gas. When an earthquake happens and breaks part of the crust, the gas is released at an extremely high speed, pushing mud up to the surface, according to National Geographic. But only earthquakes that are extremely powerful can cause mud volcanoes to push up enough mud to produce islands -- and this 7.7-magnitude earthquake in Pakistan was strong enough. The island is about 20 meters high, up to about 90 meters wide and 30 meters long, nearly the size of a soccer field. In fact, mud volcano islands aren't new. This is the fourth island of this kind in the region since 1945. But those islands usually didn't last long. "It will probably be gone within a couple of months," said Bill Barnhart, a researcher with the US Geological Survey. After all, "it's just a big pile of mud that was on the seafloor that got pushed up". According to the article, islands formed by mud volcanoes   _  .
A. are quite difficult to locate
B. usually disappear after a short period of time
C. are formed by hot lava from volcanoes
D. used to be part of the Earth's crust
Answer: