for 2 hours vs Until 8 o'clock

[vkhu]

This one is from one of my test.
"How long will your homework take you?"
".........."

A for 2 hours
B Until 8 o'clock

Either of those answers seem plausible for me. Why is it that A is the only correct answer?

EDIT: It's from a book published by Cambridge. I found the question at this site. The answer is marked in red:
http://violet.vn/MAC-XUAN-TIEN/present/show?entry_id=4100215

 

[5jj]

A, with 'for' is not correct; B is possible.

 

[vkhu]

A, with 'for' is not correct; B is possible.

Are you absolutely sure? I looked that question up and every source I found said A is the one ("for" included).

 

[SoothingDave]

Yes, he's sure. You can say "2 hours," but not "for 2 hours."

 

[Rover_KE]

Yes, he's sure. You can say "2 hours," but not "for 2 hours."

I'm sure, too.

The book has got it wrong.

Rover