Which of these is a non-linear ODE?
y′+y=0y' + y = 0y′+y=0
y′+sin(y)=0y' + \sin(y) = 0y′+sin(y)=0
y′+xy=x2y' + xy = x^2y′+xy=x2
y′′+2y′+y=0y'' + 2y' + y = 0y′′+2y′+y=0