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
考查定语从句。定语从句when everything seems tough修饰先行词periods，因为定语从句everything seems tough很完整，periods表示时间，所以使用关系副词when