They tired to think of a plan （）they could fulfill their task ahead of timeA for whichB By which请说说区别
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
选: B By which
which引导定从, By which在定从中作方式状语,"凭借/依靠/通过这个计划"
而 for which表示目的, "为了这个计划",显然不合题意.
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.