If $a$ and $b$ are arbitrary constants, then the solutions to the ordinary differential equation

$\dfrac{d^{2}y}{dx^2} – 4y = 0$

is

1. $y = ax + b$
2. $y = ae^{-x}$
3. $y = a\sin2x + b\cos2x$
4. $y = a\cosh2x + b\sinh2x$