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

混凝土搅拌机上有2个尘源点：一是向料斗上加料时飞起的粉尘；二是料斗向拌筒中倒料时，从进料口、出料口飞起的粉尘。（）