在工艺管道及仪表流程图中，是由图中的（）反映实际管道的粗细的。

A、管道标注；

B、管线粗细；

C、管线虚实；

D、管线长短。

Packet-switching wireless networks are preferable (71) when transmissions are (72) because of the way charges are (73) per packet. Circuit-switched networks are preferable for transferring large files or for other lengthy transmissions because customers are (74) for the (75) of time they use the network.

A．long

B．short

C．large

D．small