It would take much more proofreading to make sure the questions aren't ambiguous. Look over ______. What am I thinking? U = {these, there, this,....}

I thought that as well. Each question would need a good degree of context.

Agreed. If you're going the grammar route, why distort the words at all?

I was thinking of giving 5 or so sentences and having checkboxes for whether they're grammatically correct or not. I feel that even people that don't exactly know the correct usage can still get the OP's test right because one "looks" better than the other, whereas with mine there's only one choice presented.