Which of these is not a linear differential equation?
y′=y2y' = y^2y′=y2
y′+sin(x)y=0y' + \sin(x)y = 0y′+sin(x)y=0
y′+y=0y' + y = 0y′+y=0
y′′+y=xy'' + y = xy′′+y=x