Which of the following best explains the Doppler effect?
a. Bella watches a boat speed through the lake and notices the circular wave crests overlapping.
b. Bella watches drops of water splash into a pond and ripples increase in size.
c. Bella hears a train whistle but notices that it sounds different coming towards her than away from her.
d. Bella hears a shark crack after an airplane moves faster than the speed of sound.