评价疫苗免疫预防效果常用效果指数，其计算方法是（）

A.对照组发病率/接种组发病率

B.接种组发病率对/照组发病率

C.（对照组发病率-接种组发病率）/接种组发病率

D.（接种组发病率-对照组发病率）/接种组发病率

E.（对照组发病率-接种组发病率）/对照组发病率

Routing protocols use different techniques for assigning (37) to individual networks. Further, each routing protocol forms a metric aggregation in a different way. Most routing protocols can use multiple paths if the paths have an equal (38) Some routing protocols can even use multiple paths when paths have an unequal cost. In either case, load (39) can improve overall allocation of network bandwidth. When multiple paths are used, there are several ways to distribute the packets. The two most common mechanisms are per-packet load balancing and per-destination load balancing. Per-packet load balancing distributes the (40) across the possible routes in a manner proportional to the route metrics. Per-destination load balancing distributes packets across the possible routes based on (41)

38（）

A.user 

B.distance 

C.entity 

D.cost