After students have been assigned a homework set, if you discover that something is wrong with one of the questions (it's way too hard, there's a bug that you aren't able to fix immediately, etc.), you should use Mark Correct. Under these circumstances, if you use Delete It you might mess up the gradebook database. If you use Mark Correct instead, you won't mess up the gradebook database; however, you will "water down" your grades a tiny amount.
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