从句先行词疑问Write a letter to Q12 in which you explain friendship and love.中为什么要用in which?而不是用which?可以用that代替in which吗?

从句先行词疑问Write a letter to Q12 in which you explain friendship and love.中为什么要用in which?而不是用which?可以用that代替in which吗?
从句先行词疑问
Write a letter to Q12 in which you explain friendship and love.中为什么要用in which?而不是用which?可以用that代替in which吗?

从句先行词疑问Write a letter to Q12 in which you explain friendship and love.中为什么要用in which?而不是用which?可以用that代替in which吗?
这句话可以这样分解一下：
Write a letter to Q12 .
You explain friendship and love in the letter.
in which 代替的就是in the letter,不能用which替换in which.
可以用that替换in which,这个我也说不上为什么,好像就是习惯、规定而已,that可以取代 “介词+which”.