在UNIX操作系统中，若用户键入的命令参数的个数为1时，执行cat$1命令；若用户键入的命令参数的个数为2时，执行cat＞＞$2＜$1命令。请将下面所示的Shell程序的空缺部分补齐。  case(51)in  1)cat$1;;  2)cat＞＞$2＜$1;;  *)echo#'default……'  esac (51)

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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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