Car a Moves at Speed V Toward Car B,which Is

Multiple Choice

Car A moves at speed v toward car B,which is at rest.The frequency of car A's horn is observed by car B to be f.What is the frequency of car A's horn as heard by an observer in car B if car A is at rest and car B moves at speed v toward car A (assume there is no wind) ?

A) f
B) greater than f
C) less than f
D) either greater than or less than f
E) 2f