Sometime early in the next century, human beings will move to Mars. They will live there for about a year, and then will be replaced with another group of pioneers. Building the base on Mars will advance our knowledge of the solar system and aid in our understanding of the earth. We already know that Mars resembles the earth in many aspects: general size, presence of water, length of day, range of temperatures. These resemblances have caused many people to consider a centuries-long project: to terraform Mars. Terraforming means _ a planet's surface so that Earth's life forms can survive there. This concept, previously found only in science fiction is now being seriously considered by scientists. Terraforming Mars is theoretically simple: add nitrogen and oxygen to the atmosphere; pump water to the surface; and add the earth's plants and animals in the order in which they developed on Earth. But it will take at least 300 years. Some people think that such a project is too huge for humans to undertake, but there are very good reasons to make the attempt. The earth now contains some 6 billion people, and no one has any idea of how many humans the earth can support. Our very existence and numbers are threatening many other species. We also have had some experience with terraforming our own planet: altering the landscape, the atmosphere and the climate. Currently terraforming earth has become a wiser activity as we try to control global warming, air and water pollution, and preserve some natural living places. While the possibility of such a project is small, it is not impossible .Even if earth -bound societies come and go in the next 300 years, the project can continue through the work of the Mars settlers without the need for constant backing from the earth. The future existence of all the people in our world may very well depend upon our ability to terraform Mars. According to the passage the main purpose of terraforming Mars is to   _  .
do some scientific research work find out its similarity to earth avoid the dying away of many other species find on Mars living place for the increasing human beings
Question: Sometime early in the next century, human beings will move to Mars. They will live there for about a year, and then will be replaced with another group of pioneers. Building the base on Mars will advance our knowledge of the solar system and aid in our understanding of the earth. We already know that Mars resembles the earth in many aspects: general size, presence of water, length of day, range of temperatures. These resemblances have caused many people to consider a centuries-long project: to terraform Mars. Terraforming means _ a planet's surface so that Earth's life forms can survive there. This concept, previously found only in science fiction is now being seriously considered by scientists. Terraforming Mars is theoretically simple: add nitrogen and oxygen to the atmosphere; pump water to the surface; and add the earth's plants and animals in the order in which they developed on Earth. But it will take at least 300 years. Some people think that such a project is too huge for humans to undertake, but there are very good reasons to make the attempt. The earth now contains some 6 billion people, and no one has any idea of how many humans the earth can support. Our very existence and numbers are threatening many other species. We also have had some experience with terraforming our own planet: altering the landscape, the atmosphere and the climate. Currently terraforming earth has become a wiser activity as we try to control global warming, air and water pollution, and preserve some natural living places. While the possibility of such a project is small, it is not impossible .Even if earth -bound societies come and go in the next 300 years, the project can continue through the work of the Mars settlers without the need for constant backing from the earth. The future existence of all the people in our world may very well depend upon our ability to terraform Mars. According to the passage the main purpose of terraforming Mars is to   _  .
A. do some scientific research work
B. find out its similarity to earth
C. avoid the dying away of many other species
D. find on Mars living place for the increasing human beings
Answer: