Pepper, the Space Cat, Travels Toward the Centauri Star System

Pepper, the space cat, travels toward the Centauri star system 4.5 light-years away at a speed of 0.90 c. He sends back a transmission on his transmitter at a frequency of 31.20 MHz. The frequency received on Earth is

A) 7.158 MHz.
B) 31.20 MHz.
C) 68.00 MHz.
D) 136.0 MHz.