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

若一直角三角形的周长与面积的数值相等，且两直角边长之和为14，则该三角形的面积是（）。

A．20

B．24

C．12

D．6．2