Which of the following is a second-order linear differential equation?
y′+y=xy' + y = xy′+y=x
y′′+y=xy'' + y = xy′′+y=x
(y′)2+y=0(y')^2 + y = 0(y′)2+y=0
y′+(y′)2=xy' + (y')^2 = xy′+(y′)2=x