Read the following paragraph and identify its main idea by filling in the blank. 1 Creatures that are very sensitive to the changes in the air before a storm can “predict” a change in the weather. 2 Birds, for example, sense the pressure change and fly lower. 3 Low-flying birds, then, indicate that rain is coming. 4 Similarly, houseflies detect this change and move indoors to avoid the downpour. 5 And cats are known to groom themselves just before a storm. 6 In doing so, they are reacting to the static electricity that enters the air before a thunderstorm. 7 The electricity separates their fur and makes them feel dirty, so they lick themselves to make the fur smooth and “clean” again. The main idea is sentence____. ( 请写出句子的序号,例如: 1)