设函数y=f(x) 在(0,+无穷)内有界且可导,则( ) A.当( x →+无穷)lim f(x) =0 时,必有( x →+无穷)lim f'(x) =0 . B.当( x →+无穷)lim f'(x)存在时,必有( x →+无穷)lim f'(x) =0 . C.当(x→0^+) lim f(x) =0时,必有(x→0^+) lim f'(x) =0. D.当(x→0^+) lim f'(x)存在时,必有(x→0^+) lim f'(x) =0.(怎么判断)