设f(x),g(x)在(a,b)上可导,且f'(x)>g'(x),则当a<x<b时有 ()A.f(x)>g(x)B.f(x)<g(x)C.f(x)+g(a)>g(x)+f(a)D.f(x)+g(b)>g(x)+f(b)