Let y=m sin r x+n cos r x. What is the value of (d^2 y)/(d x^2) ?

$y=m \sin r x+n \cos r x$

$\frac{dy}{dx} = mr \cos rx - nr \sin rx$

$\frac{d^2 y}{d x^2} = -mr^2 \sin rx - nr^2 \cos rx = -r^2y$

Option: 4