Good question. (C) and (D) look similar but they mean very different things. It's hard to explain, but maybe some examples of how (C) and (D) are used will be helpful. Here are some examples of the two different phrases. Might as well: "You might as well go out tonight - you won't get any studying done anyway." "If you keep feeding that cat, you might as well let it live in the house." Might well: "If you put a Coke in a microwave, it might well explode." "John might well be the tallest man in town." "Might as well" means that one choice is just as good as another. "Might well" means that something is possible or likely. I hope this helps!