Which of these is a homogeneous second-order linear ODE?
y′′+4y=0y'' + 4y = 0y′′+4y=0
y′+y=0y' + y = 0y′+y=0
y′′+4y=exy'' + 4y = e^xy′′+4y=ex
y′′+y2=0y'' + y^2 = 0y′′+y2=0