要组织一次篮球联赛，赛制为单循环形式（每两队之间都赛一场），计划安排21场比赛，应邀请______个球队参加比赛．

A load line of a ship is assigned by the（）.

A.minerals management service

B.Department of energy

C.corps of engineers

D.a recognized classification socitey approved