【题目】I don't become a serious climber until the fifth grade, ______ I went up to rescue a kite that was stuck in the branches of a tree.
A.when
B.where
C.which
D.why

【答案】A
【解析】句意：一直到五年级我才成为一名认真的攀爬者，那个时候我为了拿回挂在树梢上的风筝而爬上了树。the fifth grade先行词，定语从句缺少状语，故用when来引导，故选A项。