单选题
Exercise ______ involves play and recreation，and relieves nervous tension and mental fatigue in so doing, is not only pleasant but beneficial.
A、which
B、where
C、when
D、why
【正确答案】 A
【答案解析】 从语法上分析，主句主语exercise的谓语应该是is not only pleasant but beneficial，中间显然应该是一个定语从句，定语从句所缺少的是主语，故应选A。
