逗号后面为什么用that而不用which

请下面这道题，逗号后面为什么用that而不用which?

If a book is in English, -------- means slow progress for you.

A. as B. which C. what D. that

此题容易误选 A或B，想当然地根据空格前的逗号认为这是非限制性定语从句。这样分析的同学，主要是忽略了前一句句首的 if。其实此题应选D，逗号前是一个条件状语从句，逗号后是该状语从句的主句，that在此代表前文所述的情况，用作主句的主语。

如果一本书是英文的，那对你来说意味着进展缓慢。

同样地，下面几题也应选 that，而不选 which:

(1)lf he's only interested in your looks, ---------- just shows how shallow he is

A.as B. which C. what D. that

(2) lf you want to go, -------- is quite all right with me.

A. that B. which C. and it D.so

(3)When l say two hours, ----------- includes time for eating

A.as B. which C. what D. that

(4)lf you want a double room, -------- will cost another ￡ 15.

A.as B. which C. what D. that

(5)lf you have the money, -------- will be OK

A. as B. which C and it D. that

下例也选 that 而不选 which，其中 that 表示“那”:

Unless I’m very much mistaken, -------- is my watch you're wearing!

A. as B. which C. what D. that