@GlamRockKitties Let's put in words for X and A, and hopefully that'll make the difference more clear. If you replace X with The President and A with Johnny, then (A) says essentially "if a person is the President, then he is not Johnny." Reversing in it would be "if a person is Johnny, then he is not the President." The difference with (C) is that it says "if a person is NOT Johnny, then he is the President" and "if a person is NOT the President, then he is Johnny." Having the "NOT" on one side of the equation or the other makes a tremendous difference to the meaning.
Personally, I test how I've diagrammed question by putting it into a simple if-then statement like I did above. If my if-then statement says the same thing as the original statement (albeit in different words) then I've diagrammed it correctly. If it doesn't, then I've got something mixed up.
