A moitié, etc

Question

Nous n'utilisons qu' ________ du sac de riz. We're using only one half of the bag of rice.
Why is the correct answer à moitié, which I take to mean halfway, an adverb?un demià la demiune demieune moitié

Accepted Answer

Lori, 
the quiz question that I see that matches the one you have included has a drop down response list of 4 choices, which also match those at the end of your post.
The only correct choice on that list is ‘ une moitié ‘ and the only one with ‘ à ‘ included is the incorrect ‘ à la demi ‘ . 
If you have seen the same example elsewhere with an incorrect answer, it is best to use the ‘ report it ‘ button from the quiz directly, as it can be checked more easily.