[Grammar] Peter's jacket cost ..... Jack's

[atabitaraf]

Peter's jacket cost .........Jack's.
a. twice more than
b. two times more as
c. twice as much as
d. twice more as

Here above is the test I found in Digest (Trudy Aronson)
I didn't find it out why 'c' is written as the answer. I guessed first 'a' must be the answer.
Could you please help? Thanks.

 

[MikeNewYork]

C is correct.

 

[atabitaraf]

C is correct.

Yes, but I couldn't find it why. Could you please help?
Thanks in advance.

By the way, I couldn't solve it why the verb 'cost' isn't 'costs'.

It's a difficult test, isn't it?

 

[Matthew Wai]

I couldn't solve it why the verb 'cost' isn't 'costs'.

It happened in the past, so the simple past 'cost' is used.

I guessed first 'a' must be the answer.

I take 'a' to mean 'three times as much as', but I am not a teacher.