You can draw a picture, in fact, the AB edge has been set, and the edge CD really moves on the straight line X=3.

First of all, we can consider a special case, that is, a rectangle, which can be counted as 6.

And when c is (3,5) d (3, 1), it can be counted as 9.

So you can be sure that the answer is D.

Calculate 8 by yourself. Hope to adopt. OVO