A Convenience Store Clerk Observes a Shopper 4 M in Front

A convenience store clerk observes a shopper 4 m in front of a convex mirror. The image of the shopper is located 1.5 m behind the mirror. The focal length of the mirror is

A) "-2.5 m."
B) "-2.4 m."
C) "-0.67 m."
D) "-1.1 m."
E) "none of the above."