How will our continuously growing population affect our way of life, our environment, even our planet? No matter how you look at it, we will face 41_extremely serious problem. By the middle of the 21st century, we 42_(use) up all the oil that drives our cars if at the rate we’re going. Even if scientists develop some new 43_(way)of feeding the human race, the crowded conditions on the earth will make 44_necessary for us to look for open space somewhere else. But none of the planets in our solar system are able 45_(support) life at present. One possible solution 46_the problem, however, has recently been suggested by an American scientist, Professor Carl Sagan. His idea is 47_before the earth’s resources are almost exhausted, it will be 48_(total)possible to change the atmosphere of Venus（金星）and so create a new world almost as large as earth itself.
However, 49_his idea is turned into reality, humans have a long way to go. On the one hand, Venus is much 50_(hot)than the earth; on the other hand, there is only a tiny amount of water there.
41．an 42．will have used 43．ways 44．It 45．to support
46．to 47．that 48．totally 49．before 50．hotter
