Two big mirrors A and B are fitted side by side on a wall. A man is standing at such a distance from the wall that he can see the erect image of his face in both the mirrors. When the man starts walking towards the mirrors, he finds that the size of his face in mirror A goes on increasing but that in mirror B remains the same (a) Mirror A is concave and mirror B is convex (b) Mirror A is plane and mirror B is concave (c) Mirror A is concave and mirror B is plane (d) Mirror A is convex and mirror B is concave