11. A point source of light is 60 cm from a screen and it kept at the focus of a concave mirror which reflects light on the screen. The focal length of the mirror is 20 cm. The ratio of average intensities of the illumination on the screen when the mirror is present and when the mirror is removed is:
(A) 36:1 (B) 37:1 (C) 49:1 (D) 10:1
