Which of these equations is a first-order, linear, homogeneous ODE?
y′+2y=1y' + 2y = 1y′+2y=1
y′+x2y=0y' + x^2y = 0y′+x2y=0
y′+y2=0y' + y^2 = 0y′+y2=0
y′′+y=0y'' + y = 0y′′+y=0