They tired to think of a plan （）they could fulfill their task ahead of time
问题描述：
They tired to think of a plan （）they could fulfill their task ahead of time
A for which
B By which
请说说区别
答
答案：A
解析：这是一个定语从句,用which引导,主要区分前面的介词,for表示目的,而by一般指通过.方法.
翻译：他们努力思考出一个计划,以至他们能够提前完成任务.