2009年12月大学英语四级模拟试题(2)

　　Questions 86 to 90 are based on the following passage:
　　What makes it rain? Rain falls from clouds for the same reason any-thing falls to Earth. The Earth's gravity pulls it. But every cloud ismade of water droplets or ice crystals. Why doesn't rain or snow fallconstantly from all clouds? The droplets or ice crystals in clouds areexceedingly small. The effect of gravity on them is minute. Air currentsmove and lift droplets so that the net downward displacement (¨°???￥) iszero, even though the droplets are in constant motion.
　　Droplets and ice crystals behave somewhat like dust in the air madevisible in a shaft of sunlight. To the casual observer, dust seems to actin a totally random fashion, moving about chaotically without fixed direc-tion. But in fact dust particles are much larger than water droplets andthey finally fall. The cloud droplet of average size is only 1/2500 inchin diameter. It is so small that it would take sixteen hours to fall halfa mile in perfectly still air, and it does not fall out of moving air atall. Only when the droplet grows to a diameter of 1/125 inch or largercan it fall from the cloud. The average raindrop contains a million timesas much water as a tiny cloud droplet. The growth of a cloud droplet toa size large enough to fall out is the cause of rain and other forms ofprecipitation. This important growth process is called"coalescence."
　　86. What is the main topic of the passage?
　　A) The mechanics of rain.
　　B) The climate of North America.
　　C) How gravity affects air current.
　　D) Types of clouds.
　　87. The word "minute"in Line 5 is closest in meaning to which for the
　　following?
　　A) Second. B) Tiny. C) Smooth. D) Predictable.
　　88. Why don't all ice crystals in clouds immediately fall to Earth?
　　A) They are balanced by the pressure of rain droplets.
　　B) The effect of gravity at high altitude is random.
　　C) They are kept in the sky by air currents.
　　D) They are moving about without fixed directions.
　　89. What can be inferred about drops of water larger than 1/125 inch　in diameter?
　　A) They never occur.
　　B) They are not affected by the force of gravity.
　　C) In still air they would fall to earth.
　　D) In moving air they fall at a speed of thirty-two miles per hour.
　　90. In this passage, what does the term "coalescence" (in Paragraph 2)refer to?
　　A) The gathering of small clouds to form larger clouds.
　　B) The growth of droplets.
　　C) The fall of raindrops and other precipitation.
　　D) The movement of dust particles in the sunlight.
　　WRITING
　　Directions: For this part, you are allowed 30 minutes to write a compositionon Reading -- The Best means To Acquire Knowledge in three paragraphs. Youare given the main idea of each paragraph in Chinese. You are required todevelop these ideas in English. Your composition should be no less than 120words. Remember to write clearly.