连铸保护渣的理化性能有哪些？
参考答案：
连铸保护渣的理化性能有：化学成份、熔化温度、粘度、熔化速度、铺展性和水份。
金、银首饰消费税在零售环节征收，其纳税义务发生时间为收讫销货款或取得索取销货款凭据的当天。( )
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．to
B．for
C．than
D．only
