[单项选择]

If the population of the earth goes on increasing at its current rate, there will eventually not be enough resources left to sustain life in the planet. By the middle of the 21 st century, if present trends go on, we will have used up all the oil that drives our cars, for example. Even if scientists develop new ways of feeding the human race, the crowded conditions on the earth will make it necessary for us to look for open space somewhere else. But none of the other planets in our solar system are capable of supporting life at present. One possible solution to the problem, however, has recently been suggested by an American scientist, Professor Cart Sagan.
　　Sagan believes that before the earth’s resources are completely exhausted it will be possible to change that atmosphere of Venus and so create a new world almost as large as the earth itself. The difficulty is that Venus is much hotter than the earth and there is only a tiny amount of water there.
　　Sag
A. Lack of oil.
B. Lack of food.
C. Lack of space.
D. Lack of resources.

[单项选择]If the population of the earth goes on increasing at its present rate, there will eventually not be enough resources left to maintain life on the planet. By the middle of the 21st century, if present trends continue, we will have used up all the oil that drives our cars. Even though scientists develop new ways of feeding the human race, the crowded conditions on earth will make it necessary for us to look for open space somewhere else. But none of the other planets in our solar sys- tem are capable of supporting life at present. One possible solution to the problem, however, has recently been suggested by American scientist, Professor Carl Sagan.
Sagan believes that before the earth’s resources are completely exhausted it will be possible to change the atmosphere of Venus. The difficulty is that Venus is much hotter than the earth and there is only a tiny amount of water there.
Sagan suggests that algae(藻类) organisms(生物，有机体) that can live in extremely hot or cold atmospheres
A. By the end of the twenty-second century.
B. By the end of the twenty-first century.
C. By the middle of the twenty-first century.
D. By the middle of file twenty-second century.