Which of these is NOT a first-order linear differential equation?
y′+5y=0y' + 5y = 0y′+5y=0
y′+xy=x2y' + xy = x^2y′+xy=x2
y′+y2=1y' + y^2 = 1y′+y2=1
y′+exy=sin(x)y' + e^x y = \sin(x)y′+exy=sin(x)