请说明喷气发动机与活塞式发动机相比，他的优点在哪里？
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Which two of these are characteristics of multicast routing？ （）
A. multicast routing uses RPF
B. multicast routing is connectionless.
C. In multicast routing， the source of a packet is known.
D. When network topologies change， multicast distribution trees are not rebuilt， but use the originalpath
E. Multicast routing is much like unicast routing， with the only difference being that it has a a groupof receivers rather than just one destinatio
课程设计的定义可以分为两类：一类侧重于（）层面上的课程设计，另一类侧重（）层面上的课程设计。
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