Cheer up! Everyone may have periods in their liveseverything seems tough.A. whichB. where

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。