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