问题补充：
Some of the students are nearsightedthey spend too much time watching TV or playing online games.
A. when
B. though
C. because
D. unless
答案：
A. when
B. though
(答案→)C. because
D. unless
解析：句意：一些学生近视眼，因为他们花费太多的时间在看电视和网络游戏上。A当连接时间状语从句B虽然，转折连词，D如果不，条件状语从句。故选C，因为，连接原因状语从句。
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