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 and at the same time produce oxygen, should be bred in condition similar to those on Venus. As soon as this has been done, the algae will be placed in small rockets. Space- ship will then’ fly to Venus and fire the rockets into the atmosphere. In a fairly short time, the algae will break down the carbon dioxide(二氧化碳) into oxygen and carbon. When the algae have done their work, the atmosphere will become cooler, but before man can set foot on Venus it will be necessary for the oxygen to produce rain. The surface of the planet will still be too hot for man to land on but the rain will eventually fall and in a few years something like earth will be reproduced on Venus. If the population of’ the earth goes on increasing at its present rate, when will we use up all the oil that drives our cars

A. By the end of the twenty-second century.
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.