问题
选择题
Cheer up! Everyone may have periods in their lives everything seems tough.
A．which
B．where
C．when
D．that
答案
答案：C
题目分析：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。