重点讲解：
1. a waste of
浪费；
eg. It is a waste of time to listen to his insignificant talk.
听他那些废话纯属浪费时间。
eg. I've finished with gambling it's a waste of money.
我已经戒赌了——赌博完全是浪费钱财。
2. set off
点燃；
eg. If you drop a lighted cigarette, you could set off a fire.
如果你把一根点燃的香烟掉在地上，就有可能引起火灾。
eg. He likes to set off firecrackers during Diwali.
他喜欢在排灯节放鞭炮。
3. lead to
招致；致使；导致；
eg. There are recognised black spots in marriages which can lead to trouble.
婚姻中有一些公认的容易引发问题的雷区。
eg. Early success may lead to megalomania.
少年得志可能导致妄自尊大。
