People have known how to make ethanol since the dawn of civilisation, if not before.
如果不能说在人类文明开始之前人们就学会制造乙醇，那这也是人类文明刚刚出现就有的事了。
Most migrant worker families must therefore send their children back home when they reach junior high school age, if not before.
所以，大多数的农民工家庭只好尽早把他们的孩子送回老家读初中。
Most migrant worker families must therefore send their children back home when they reach junior high school age, if not before.
所以，大多数的农民工家庭只好尽早把他们的孩子送回老家读初中。
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