21. In the long run, the most difficult problem caused by population growth on earth will probably be the lack of______.
A. food B. oil C. space D. resources
22. Carl Sagan believes that Venus might be colonized from earth because______.
A. it might be possible to change its atmosphere
B. its atmosphere is the same as the earth's
C. there is a good supply of water on Venus
D. the days on Venus are long enough
23. On Venus there is a lot of______.
A. water B. carbon dioxide
C. carbon monoxide D. oxygen
24. Algae are plants that can______.
A. live in very hot temperatures B. live in very cold temperatures
C. manufacture oxygen D. all of the above
25. Man can eventually land on Venus only when______.
A. the algae have done their work B. the atmosphere becomes cooler
C. there is oxygen D. it rains there
