逗號后面為什么用that而不用which

請下面這道題，逗號后面為什么用that而不用which？

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

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

此題容易誤選 A或B，想當然地根據空格前的逗號認為這是非限制性定語從句。這樣分析的同學，主要是忽略了前一句句首的 if。其實此題應選D，逗號前是一個條件狀語從句，逗號后是該狀語從句的主句，that 在此代表前文所述的情況，用作主句的主語。同樣地，下面幾題也應選 that，而不選 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

下例也選 that 而不選 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