An experiment is conducted to determine how the arm length of a pendulum effects its period. The experimental design calls for varying the length of the arm and the mass of the bob. Which is true about this experiment, based on good experimental design?

a. Only the arm length should be changed. The mass should remain the same.
b. The force applied to the mass should be varied for trial.
c. The material the arm is made of should be varied for each trial.
d. Only the mass should be changed. The arm length should remain the same