he must be near-sighted _ he wears eys-glasses a.for b.becau

答案为 A for
because 是指直接原因,导致近视的直接原因可不是因为戴眼镜
for 在这里虽然在汉语中可以译为“因为”,从逻辑上来讲,它其实是表结果的,即 【因为近视,他才戴眼镜的】,这种可以【倒因为果】的情况英语中只能用for.
再如：
He must have failed in the exam,for he is very sad.
比较：
He must have failed in the exam,because he is very lazy.

a，for是说明原因但没有because那么清楚和强烈，because一般是用 because。。。。。

这里选择B.because, for有时候也可以和because互用，但for还带有"为了...,为..."的意思，所以此处不可以用for.
He wears eye-glasses for near-sighted.或者He wears eye-glasses because of near-sighted.

b.because 用for要这样he must be near-sighted ，for he wears eys-glasses