问题补充：
Cheer up! Everyone may have periods in their liveseverything seems tough.
A. which
B. where
C. when
D. that
答案：
A. which
B. where
(答案→)C. when
D. that
解析：考查定语从句。which/that作主语或宾语；where作地点状语；when作时间状语；先行词是periods“时期”，指时间，而且定语从句不缺少成分，因此用when引导定语从句。故选C。
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