阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入一个适当的词或使用括号中词语的正确形式填空，并将答案填写在答题卡的相应位置上。
If the population of the Earth keeps on increasing at its present rate, there will eventually not be enough resources ________(leave) to support life on the planet. By the middle of the 21^st century, if present trends continue, we_________（use）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 ______ necessary for us to look for open space somewhere else. But______ of the other planets in our solar system are capable of supporting life at present. One possible solution______ the problem, _________, has recently been suggested by an American scientist, Professor Carl Sagan. Sagan believes _______ before the Earth’s resources are _______(complete) exhausted it will be possible to change the atmosphere of Venus and therefore create a
new world almost as large as the Earth    .The difficulty is that Venus is much hotter than the Earth. ________, there is only a tiny amount of water there.