某系统由A、B、C 3个子系统串联而成，它们的寿命均服从指数分布，其中子系统A、B的MTBF分别是200h、400h。现要求系统的MTBF在100h以上，则子系统C的MTBF至少为(3)h。

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Multipurpose　Internet　MaiI　Extension　(MIME)　is　a(71)document　messaging　standard　in　the　Internet　enviroment,　with　MIME,　users　can　send(72)E-mail　messages　that　include　audio,　video,　graphices,　and　text　to　any　other　user　of　a　TCP/IP　network.　Richtext　information　can　also　be(73)into　messages.　It　defines　the　fonts,　formats,　and(74)features　of　a　document　so　the　document　can　be　easily(75)on　many　different　types　of　systems.

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Packet-switching　wireless　networks　are　preferable(66)when　transmissions　are(67)because　of　the　way　charges　are(68)per　packet.　Circuit-switched　networks　are　preferable　for　transferring　large　files　or　for　other　lengthy　transmissions　because　customers　are(69)for　the(70)of　time　they　use　the　network.

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