Do you remember a certain occasi

Do you remember a certain occasion ______ you were in trouble and at that moment I gave you a hand.
A、where
B、which
C、why
D、when
【正确答案】：D
【题目解析】：考查定语从句。根据后面的时间短语at that moment可知occasion指时间，故要使用关系副词when，相当于at which time。