口腔白色角化病与白斑最主要的区别是（）

A.前者病因多为机械性刺激因素，后者则为物理化学因素

B.前者损害呈灰白色，后者呈白色

C.前者损害大，后者小

D.前者质软，后者质硬

E.前者病理变化为单纯性的上皮增生，后者则为上皮异常增生

Specific duties are levied ______ some physical unit such as dollars per bushel, per kilo, or per metre and ad valorem duties are calculated as a percent of the value of the goods.

A．on the basis of

B．with the basis of

C．on the base of

D．with the base of