Two cars are on the same straight road. Car A moves east at 55 mph and sounds its horn at 625 Hz. Rank from highest to lowest the frequency observed by Car B's driver in each of the following cases. (Use the following as necessary: a, b, c, and d. Use only ">" or "=" symbols. Do not include any parentheses around the letters or symbols. Let "behind" mean car B is west of car A, and "in front of" mean car B is east of car A.)

• (a) Car B is moving east at 70 mph and is behind car A.
• (b) Car B is moving east at 70 mph and is in front of car A.
• (c) Car B is moving west at 70 mph and is behind car A.
• (d) Car B is moving west at 70 mph and is in front of car A.