第21题:[单选题]( )You must take 2 tugs fore and 1 tug aft A.It is possible for you to take 3 tugs fore and aft B.It is necessary for you to take 3 tugs fore and aft C.It is possible for you to take 2 tugs fore and aft D.It is necessary for you to take 2 tugs fore and aft 参考答案:B