英语四级听力全真模拟题(二)_大学英语听力

英语四级听力全真模拟题(二)

　　Droplets and ice crystals behave somewhat like dust in the air made visible in a shaft of sunlight. To the casual observer, dust seems to act in a totally random fashion without fixed direction. But in fact dust particles are much larger than water droplets and they finally fall. The cloud droplet of average size is only 1/2500 inch in diameter. It is so small that it would take sixteen hours to fall a mile in perfectly still air, and it does not fall out of moving air at all. Only when the droplet grows to a diameter of 1/125 inch or larger can it fall from the clouds. The average raindrop contains a million times as much water as a tiny cloud droplet. The growth of a cloud droplet to a size large enough to fall out is the cause of rain and other forms of precipitation. This important growth process is called coalescence。

　　Questions 26 to 28 are based on the passage you have just heard。

　　26. What is the main topic of the passage?

　　27. Why dont all ice crystals in clouds immediately fall to Earth?

　　28. In this passage, what does the term coalescence refer to?

　　Passage Two

　　Good locks on all outside doors and first floor windows are essential to a homes security and will go a long way towards discouraging burglars. But an added protective device that is increasingly being used in homes is the burglar alarm system, which sounds a loud bell when someone tries to break in. Although this will not always prevent entry by a professional burglar, it is often enough to scare off thieves who account for a high percentage of residential break-ins。