发展容错技术可提高计算机系统的可靠性。利用元件冗余可保证在局部有故障的情况下系统正常工作。带有热备份的系统称为(61)系统。它是(62)，因此只要有一个子系统能正常工作，整个系统仍能正常工作。

 当子系统只能处于正常工作和不工作两种状态时，可以采用如图2(a)所示的并联模型，若单个子系统的可靠性都为0.8时，图2所示的三个子系统并联后的系统可靠性为(63)。若子系统能处于正常和不正常状态时，我们可以采用如图2(b)所示的表决模型，若图中有任何两个以上子系统输出相同时，则选择该输出作为系统输出。设单个子系统的可靠性为0.8时，整个系统的可靠性为(64)；若单个子系统的可靠性为0.5时，整个系统的可靠性为(65)。

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Ipv6 is(71)for "Internet Protocol Version 6"。Ipv6 is the "next generation" protocol design by the IETF to(72)the current version Inernet Protocol, IP Version 4. Most of today's internet uses Ipv4, which is now nearly twenty years old. Ipv4 has been remarkably resilient in spite of its age, but it is beginging to have problems. Most importantly, there is a growing(73)of Ipv4 addresses, which are needed by all new machines added to the Internet. Ipv6 fixs a number of problems in Ipv4, shch as the(74)number of available Ipv4 addresses. It also adds many improvements to Ipv4 in areas such as souting and network autoconfiguration. Ipv6 is expected to gradually replace Ipv4, with the two coexing for a number of years during a transition(75)A.

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In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet.

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