6. The figure shows four situations in which a source of sound S and a detector D are either moving or stationary. The arrows indicate the directions of motion. The speeds (when not zero) are all the same. (Note that the detector in situation ③ is stationary). Rank the situations according to the apparent frequency of the source, lowest to highest. (A) ①,②,③,④ (B) ④,③,②,① (C) ①,③,②,④ (D) ②,①,④,③ (E) ②,①,③,④