根据短文内容，在空白处填入一个适当的词，或用所给单词的适当形式填空。

    What will our life be like in future? Some scientists think many people will live in cities, so cities will be very big and (crowd). There will be more cars in big cities, and the air (pollute) will be even worse. People will cut down too many trees (build) houses, so there will be (few) trees. The earth will not be suitable to live . Some people will even have to live on space stations. Scientists will find other (planet) for people to live on. So rockets will be the most important vehicles (交通工具). But many scientists (agree). They think humans will make the earth more beautiful and people will a happy life. Who do you agree ?