Little tip, everybody.
I see a lot of questions that amount to something like "Why is this answer right?" "Why is this answer wrong?" "Why is it A and no D?"
One thing that would help we who answer such questions *really* help you is for you to try, as best as you can, to explain what *you* are thinking?
If you're not sure why the right answer is right, why do you think it might be wrong? Or why are you uncertain? If you think the wrong answer might be right, why do *you* think it is right?
Forcing yourself to explain your reasoning will do a few good things.
1). It might reveal to you where your own reasoning is flawed. You might realize what you had been overlooking, or what assumption you were bringing to the table without being aware of it.
2). It will help us know exactly what you're struggling with, and therefore how to best help you. Ideally, I don't write out a full explanation for a problem. I ask you a question that gets you thinking in a way that, hopefully, reveals the right line of reasoning. But "Why is C wrong?" just results in me writing an explanation, which isn't as helpful to you as it might seem. Reasoning is a skill, like playing a musical instrument. Reading someone else's reasoning is a bit like watching someone else play the piano. I can't say that isn't sometimes helpful, maybe? But far more helpful is playing the piano yourself.