Each side of a given polygon is parallel to either the X or the Y axis. A corner of such a polygon is said to be convex if the internal angle is 90 degree or concave if the internal angle is 270 degree. If the number of convex corners in such a polygon is 25, the number of concave corners must be
(A) 0
(B) 19
(C) 20
(D) 21
(E) 22