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

两点间高差（h）与该两点连线的长度（L）的比叫坡度i：h/L。