Consider the following two equation:

$\frac{dx}{dt}+x+y=0$

$\frac{dy}{dt}-x=0$

The above set of equations is represented by

1. $\frac{d^{2}y}{dt^{2}}-\frac{dy}{dt}-y=0$
2. $\frac{d^{2}x}{dt^{2}}-\frac{dx}{dt}-y=0$
3. $\frac{d^{2}y}{dt^{2}}+\frac{dy}{dt}+y=0$
4. $\frac{d^{2}x}{dt^{2}}+\frac{dx}{dt}+y=0$