[填空题] 传染病流行必需的3个环节是：（），（），（）。

下列哪种物质对头孢菌素C的生物合成具有诱导作用？（） ["缬氨酸","苯乙酸","苯乙酰胺","L-甲硫氨酸"] 运动治疗 接受质量限即可接受质量水平 How dose a DHCP server dynamically assign IP address to host?（） [" Addresses are allocated after a negotiation between the server and the host to determine the length of the agreement."," Addresses are assigned for a fixed period of time.At the end of period,a newquest for an address must be made ,and another address is then assigned."," Addresses are leased to host.A host will usually keep the same address by periodically contacting the DHCP sever to renew the lease."," Addresses are permanently assigned so that the host uses the same address at all times."] 学前儿童运动系统有何特点？如何做好学前儿童运动系统的卫生保健工作？. 传染病流行必需的3个环节是：（），（），（）。