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.
