A concave mirror is placed at the bottom of an empty tank with face upwards and axis vertical. When sunlight falls normally on the mirror, it is focussed at distance of `32cm` from the mirror. If the tank filled with water `(mu=4//3)` up to a height of `20cm`, then the sunlight will now get focussed at
A. 16 cm above water level
B. 9 cm above water level
C. 24 cm below water level
D. 9 cm below water level