f (x)定义在(0，＋∞)上的非负可导函数，且满足≤0，对任意的正数a，b，若a＜b，则必有 ( ▲  )

A．bf (b)≤af (a)

B．bf (a)≤af (b)

C．af (a)≤bf (b)

D．af (b)≤bf (a)