26. It is believed that the earth gets colder when ________.��
  A. volcanoes erupt��
  B. the air is polluted by modern man��
  C. the rays of the sun are turned into space��
  D. all of the above��

27. The effects of Tambora's eruption were ________.��
  A. felt mainly in the Dutch East Indias��
  B. of several day's duration worldwide��
  C. evidence of pollution's cooling the earth��
  D. immediately evident to the world's scientists��

28. The cause of cold weather in England from 1815 to 1818 was ________.
  A. decreased crop and livestock production��
  B. volcanic ash in the atmosphere��
  C. pollution caused by the Industrial Revolution��
  D. its proximity to the North sea��

29. No one realized the cause of the deterioration of the world agricultural commodity market because ________.��
  A. there was a long delay between cause and effect��
  B. the weather is beyond our comprehension��
  C. weather forecasts were inaccurate��
  D. ecologists didn't exist until modern times��

30. If, as some scientists predict, the world ends in ice, what might be the cause?
  A. Modern man's pollution of the air.��
  B. Volcanic eruptions.��
  C. Obliteration(破壞) of solar radiation.��
  D. All of the above.

Passage Three

Questions 31 to 35 are based on the following passage.

     The great advance in rocket theory 40 years ago showed that liquid�瞗uel rockets were far superior in every respect to the skyrocket with its weak solid fuel, the only kind of rocket then known. However, during the last decade, large solid�瞗uel rockets with solid fuels about as powerful as liquid fuels have made their appearance, and it is a favorite layman's(外行) question to inquire which one is “better.” The question is meaningless; one might as well ask whether a gasoline or a diesel engine is “better.” It all depends on the purpose. A liquid fuel rocket is complicated, but has the advantage that it can be controlled beautifully. The burning of the rocket engine can be stopped completely; it can be reignited(重新點(diǎn)火) when desired. In addition, the thrust can be made to vary by adjusting the speed of the fuel pumps. A solid�瞗uel rocket, on the other hand, is rather simple in construction, though hard to build when a really large size is desired. But once you have a solid�瞗uel rocket, it is ready for action at very short notice. A liquid�瞗uel rocket has to be fueled first and cannot be held in readiness for very long after it has been fueled. However, once a solid�瞗uel rocket has been ignited, it will keep buming. It cannot be stopped and reignited whenever desired (it could conceivably be stopped and reignited after a pre calculated time of burning has elapsed) and its thrust cannot be varied. Because a solid�瞗uel rocket can be kept ready for a long time, most military missiles employ solid fuels, but manned spaceflight needs the fine adjustments that can only be provided by liquid fuels. It may be added that a liquid�瞗uel rocket is an expensive device; a large solid�瞗uel rocket is, by comparison, cheap. But the solid fuel, pound per pound, costs about 10 times as much as the liquid fuel. So you have, on the one hand, an expensive rocket with a cheap fuel and on the other hand a comparatively cheap rocket with an expensive fuel.��

31. The author feels that a comparison of liquid�瞐nd solid�瞗uel rockets shows that ________.��
  A. neither type is very economical��
  B. the liquid�瞗uel rocket is best��
  C. each type has certain advantages��
  D. the solid�瞗uel rocket is best��

32. The most important consideration for manned space flight is that the rocket be ________.��
  A. inexpensive to construct��
  B. capable of lifting heavy spacecraft into orbit��
  C. easily controlled��
  D. inexpensive to operate��