防水快凝砂浆（质量比）：（）。

A、水泥100%防水剂69%水50%

B、水泥100%砂220％防水剂24％水46％

C、水泥50%防水剂69%水100%

D、水泥220％砂100%防水剂24％水46％

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．point

B．start

C．length

D．end