Which of the following must be shown to verify that the function y=f(x) is a solution of the first-order differential equation dxdy=g(x,y) on an interval I?
(a) f(x) is differentiable at every point of I.
(b) f′(x)=g(x,f(x)) at a single point of I.
(c) f′(x)=g(x,f(x)) for every x\,in\,I.
(d) ∫g(x,f(x))dx=f(x)+C for some constant C.