A point object O is kept at origin. When a concave mirror $$M_1$$ placed at $$x=6$$ cm, image is formed at infinity. When $$M_1$$ is replaced by another concave mirror $$M_2$$ at the same position, image is formed at $$x=30$$ cm, then the ratio of the focal length of $$M_1$$ to that of $$M_2$$ is
[image]

A

$$\frac{3}{4}$$

B

$$\frac{4}{3}$$

C

5

D

$$\frac{1}{5}$$