当压力水管发生直接水锤时，只有在阀门处产生最大水锤压强的关闭时间应为（）。

A.Ts=0 

B.L/a<Ts<2L/a 

C.Ts>2L/a 

D.Ts=2L/a

The Internet is a (66) network of networks which interconnects (67) ranging from the simplest personal computers to the most sophisticated mainframes and supercomputers. This network of networks is used for an extraordinary range of purposes long distance collaborations, retrieving free software and documents, (68) library catalogs, (69) in to supercomputers - the list of current Internet activities and (70) is large and the range of possible future applications is as unlimited as our imagination.

A．logging

B．going

C．coming

D．achieving