The period of a simple pendulum is measured to be 4.0 seconds in a frame of reference. Which of the following is correct when the period of the pendulum is measured by an observer moving with 0.4 times the speed of the light?

Choose one answer.
a. The time period of the pendulum remains constant.
b. The time period of the pendulum decreases.
c. The time period of the pendulum increases.
d. The time period of the pendulum decreases by half.