A multicast router may receive thousands of multicast （） every day for different groups. If a router has no knowledge about the membership status of the （） , it must broadcast all of these packets. This creates a lot of traffic and consumes （）. A better solution is to keep a list of groups in the network for which there is at least one loyal member. （） helps the multicast router create and update this list. For each group, there is one router that has the duty of distributing the （） packets destined for that group. This means that if there are three multicast routers connected to a network, their lists of groupids are mutually exclusive. A host or multicast router can have membership in a group.

A multicast router may receive thousands of multicast （） every day for different groups.

A. packets

B. errors

C. reports

D. alarms

如图所示，活塞的质量为m，缸套的质量为M，通过弹簧吊在天花板上，汽缸内封住一定质量的气体，缸套和活塞间无摩擦，活塞面积为S，大气压强为p₀，则封闭气体的压强为(  )

A．p＝p₀＋

B．p＝p₀＋

C．p＝p0－

D．p＝mg/S