Observer a Sees a Pendulum Oscillating Back and Forth in a Relativistic

Multiple Choice

Observer A sees a pendulum oscillating back and forth in a relativistic rocket and measures its period to be TA. Observer B moves along with the rocket and measures the period of the
Pendulum to be TB. What is true about these two time measurements?

A) $T _ { A } = T _ { B }$
B) $T _ { A } > T _ { B }$
C) $T \mathrm {~A} < \mathrm { T } _ { \mathrm { B } }$
D) $T$ A could be greater or smaller than $T _ { B }$ depending on the direction of the motion.