B, because the overall square pattern area is 49,

From the combination, it can be (x+y)2 or (4xy+4).

So there are (x+y)2=49 and 4xy+4=49.

That is, xy=454,

So (x-y)2=(x+y)2-4xy=49-45=4,

That is, x-y = 2；;

C, x2+y2=(x+y)2-2xy=49-2×454=532, so x2+y2=25 is wrong;

D, you can see from B that 4xy+4 = 49.

So choose C.