31．根據(jù)短文內(nèi)容及首字母提示，填寫所缺單詞。
The world is full of many interesting sounds.Some are unpleasant to our e（1）
while others are very pleasant to hear.In d（2）
life you may hear hundreds of sounds.All sounds are different.Some may be soft；others may be loud.Some sounds are high，others are l（3）
.
Some sounds are useful.W（4）
sound we can't talk or listen to each other.The ringing of clock w（5）
people up.The hooting（鳴笛聲）of a car warns people there may be d（6）
.
Some sounds are dangerous.When planes fly above the land，the sounds can break the house down.Very loud sound can even m（7）
people deaf.
We know sound travels about one kilometer in three s（8）
.In a thunder storm you see the lightning first and then hear the thunder.This is because light travels m（9）
faster than sound.
Next time you see lightning，count the number b（10）
you hear the thunder.
Divide（除）this number by 3.It will tell you how far the thunder storm is.
