For the ordinary differential equation $\frac{d^{2} y}{d x^{2}}+4 y=0$, the general solution is
- $y=c_{1} \cos 2 x+c_{2} \sin 2 x$
- $y=c_{1} \cosh 2 x+c_{2} \sinh 2 x$
- $y=c_{1} e^{2 x}+c_{2} e^{-2 x}$
- $y=c_{1} e^{2 x} \cos 2 x+c_{2} e^{-2 x} \sin 2 x$
