I don`t understand the way ___they worked out the problem A.that B.which C.why D.when

I don`t understand the way ___they worked out the problem A.that B.which C.why D.when

选A
the way作先行词之后的定语从句：
当“way”表示“方式、方法”在句中作先行词,后被一个定语从句修饰时,引导定语从句可用that ,也可用in which,或什么都不用.