see off的用法：
I went to the station to SEE them OFF.
I went to the airport to SEE them OFF.
我到车站/我到机场 送别 他们.
（常说的送人接人,see off是送人-- 看.离去）
A cat came into the back garden but the dog soon SAW it OFF.
猫进了后院（花园）,狗很快就把它赶走.
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set off的用法：
The smoke SET the fire alarm OFF.
烟雾使火警报警器响起.
We SET OFF for work at seven-thirty.
我们7点半离开（家）,（然后走路或开车或乘车)去上班.
The company SET OFF its overseas debts against it profits at home.
（.偿还它的海外债务.）
Terrorists SET OFF a car bomb in the city centre last night.
Fortunately,no-one was hurt or killed.
（.引爆.）
