Which of these is a first-order linear ODE?
y′+xy=0y' + xy = 0y′+xy=0
y′′+y=0y'' + y = 0y′′+y=0
y′+y2=1y' + y^2 = 1y′+y2=1
sin(y′)=x\sin(y') = xsin(y′)=x