第二节：语法填空
There are two main reasons   11     the temperature of the air is always      12   （change）. One reason is that the air around you moves in from somewhere else . Air usually moves in large bodies   13     (call) air masses. Some air masses move in from the north. These air masses usually have cold air that cause the temperature 14     (drop).  Air masses also move in from the south. They usually have   15      (warm) air that causes the temperature to rise.
Temperature  changes during the day or night can be great   16       small. This depends partly   17     how many clouds are in the sky.    18      there are many clouds during the night, they act like tile covers on your bed. They blanket the earth and keep the air from losing much of   19     heat. The temperature usually does not drop much during such a night. When there are many clouds during   20       day, some of the sun's energy will not reach the earth. The temperature usually does not rise much on such a day.