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。同样地，下面几题也应选 that，而不选 which：

(1) If he's only interested in your looks, _____ just shows how shallow he is.

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

(2) If you want to go, _______is quite all right with me.

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

(3) When I say two hours, _____ includes time for eating.

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

(4) If you want a double room , _____ will cost another ￡15.

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

(5) If 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