某工程计划如图6-2所示，弧上的标记为作业编码及其需要的完成时间(天)，作业F最迟开始时间需在第(10)天开始。

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The　major　problem　with　E-mail　is　that　it　is(71)easy　to　use　that　people　can　become(72)with　messages(73)they　can　possibly　answer　in　a　day.　In　addition,　mail　boxes　require　some　management　to(74)messages　or　archive　those　that　might　be　required　later.　Senders　don't　always　know　about　your　E-mail　backlog　and　often　send(75)messages.

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Communication　protocols　are(66)connection-oriented　or　connectionless,(67)whether　the　sender　of　a　message　needs　to　contact　and　maintain　a　dialog　with　the　recipient　or(68)send　a　message　without　any,　prior　connect　and　with　the　hope　that　the　recipient　receives　everything.(69).These　methods(70)the　two　ways　that　communication　is　implemented　on　networks.

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