Which of these is a first-order linear differential equation?
y′+y2=xy' + y^2 = xy′+y2=x
y′+x2y=exy' + x^2y = e^xy′+x2y=ex
y′′+y=0y'' + y = 0y′′+y=0
y′=yy' = \sqrt{y}y′=y