You Stand by the Railroad Tracks as a Train Passes

Multiple Choice

You stand by the railroad tracks as a train passes by.You hear a 1,100-Hz frequency when the train approaches,which changes to 870 Hz as it goes away.How fast is the train moving? The speed of sound in air is 340 m/s.

A) 26.6 m/s
B) 33.5 m/s
C) 37.9 m/s
D) 39.7 m/s
E) ​43.0 m/s