在潜水作业的条件下，潜水员体内矿物质代谢受到较明显影响的是（）。
A.钙
B.铁
C.钾
D.钠
E.锌
参考答案：C
Packet-switching wireless networks are preferable (41) when transmissions are (42) because of the way charges are (43) per packet. Circuit-switched networks are preferable for transferring large files or for other lengthy transmissions because customers are (44) for the (45) of time they use the net-work.
A．long
B．short
C．large
D．small
简答模型库子系统的具体功能。
