Let's find quantity A first: if you visualize a cube, imagine the square on the top. The top square has 4 sides. So does the bottom square, making 8 so far. There are also 4 vertical edges joining the two squares, for a total of 12.

Another way to figure out quantity A is this: there are 6 squares, each of which has 4 edges. So since 6x4=24, that makes 24 edges. However, every edge joins two squares, so we have counted every edge twice, so we should divide 24 by 2 to get 12 again.

Quantity B is simple. There are 6 sides to the cube, so doubling that, we get 12 again, so the answer is C.