Well, at least you are 虚心学习

I remember there was a guy in DQ who claimed he is a staticstical expert and laughing at people who are learning probability.  The way he calculate probalility shows me that the guy 知其然不知其所以然。And he does not even know that.  I meet a Ph.D in Math. He was the same - 知其然不知其所以然.

Actually, I came with answer (A) at first glance. However, after I saw the answer was (C), I realized that I might have made a mistake and I dig into it and found out that I did make a mistake by assuming it's a square at first place.