Which of the following differential equations is NOT first-order?
y′+3y=0y' + 3y = 0y′+3y=0
y′′+y=xy'' + y = xy′′+y=x
dydx=ex\frac{dy}{dx} = e^xdxdy=ex
xy′+y=1x y' + y = 1xy′+y=1