for 2 hours vs Until 8 o'clock

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

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

5jj said:

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

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

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

SoothingDave said:

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

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I'm sure, too.

The book has got it wrong.

Rover