Which one of the following differential equations has a periodic...

Options

1. A$\frac{\mathrm{d}^2 \mathrm{x}}{\mathrm{dt}^2}+\mu \mathrm{x}=0$
2. B$\frac{\mathrm{d}^2 \mathrm{x}}{\mathrm{dt}^2}-\mu \mathrm{x}=0$
3. C$\mathrm{x} \frac{\mathrm{dx}}{\mathrm{dt}}+\mu \mathrm{t}=0$
4. D$$\begin{aligned} & \frac{\mathrm{dx}}{\mathrm{dt}}+\mu \mathrm{xt}=0 \\ & \text{where } \mu > 0\end{aligned}$$