Cheer up! Everyone may have periods in their lives everything seems tough.
A. which
B. where
C. when
D. that
网友回答
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。