Suppose you travel to the moon, and you take with you two timepieces: a pendulum clock and a wristwatch that runs with a wheel and a mainspring. (The wheel and spring work, essentially, like a mass on a spring, but the wheel rotates back and forth rather than moving up and down.) Which will keep good time on the moon? 

A. Only the pendulum clock 

B. Only the wristwatch 

C. Both timepieces 

D. Neither timepiece