Image is obtained on a screen by keeping an object at $$25\ \text{cm}$$ and at $$40\ \text{cm}$$ in front of a concave mirror. Image in the former case is four times bigger than in the latter. Focal length of the mirror must be

A

$$12\ \text{cm}$$

B

$$20\ \text{cm}$$

C

$$24\ \text{cm}$$

D

$$36\ \text{cm}$$