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              Sounds are everywhere on the Earth. The world__36__many interesting sounds. Some are unpleasant to our ears while__37__are very pleasant to hear. In a single day, you probably hear __38__ sounds. All sounds are different. Some__39__loud while others may be low.

                 Some sounds are useful. __40__sounds we can’t talk or listen to each other. The ringing of the alarm clock wakes people up. The hoot(鸣笛声) of a car warns people of danger.

                Some sounds are harmful. When planes fly low__41__the land, the very loud sounds can even make people deaf.

                We know sound travels about one kilometre in three__42__. In a thunderstorm, you see the lightning first and then hear the thunder. This is because light travels__43__than sound.

              Next time you see lightning, count the number of seconds before you hear the thunder.

              Divide（除以）this number__44__ 3. This will tell you__45__kilometres away the thunderstorm is.

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