Out of the following differential equations, one that correctly represents the motion of a second's pendulum is

A

$$\frac{d^2 x}{dt^2} + \frac{x}{\pi} = 0$$

B

$$\frac{d^2 x}{dt^2} + \frac{x}{\pi^2} = 0$$

C

$$\frac{d^2 x}{dt^2} + \pi x = 0$$

D

$$\frac{d^2 x}{dt^2} + \pi^2 x = 0$$