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。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