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.