Three physics faculty members sit in three
different cars at a railroad crossing. Three trains
go by at different (constant) speeds, each
emitting the same sound. Each faculty member
uses a cell phone to record the sound of
different train. On the next day, during a faculty
meeting, they plot the frequency as a function of
time on a computer, resulting in the plots shown
in the figure. Which of the three trains had the
highest speed

A) the one represented by the solid line
(B) the one represented by the dashed line
(C) the one represented by the dotted line
(D) impossible to tell