回溯法也称为试探法。以下关于回溯法的结论中正确的是(50)。

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Software　design　is　a(71)process　.It　requires　a　certain(72)of　flair　on　the　part　of　the　designer.　Design　can　not　be　learned　from　a　book　.It　must　be　practiced　and　learnt　by　experience　and　study　of　existing　systems　.A　well(73)software　system　is　straightforward　to　implement　and　maintain　,easily(74)and　reliable　.Badly(73)software　systems　,although　they　may　work　are(75)to　be　expensive　to　maintain　,difficult　to　test　and　unreliable.

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(66)is a protocol that a host uses to inform. a router when it joins or leaves an Internet multicast group.

(67)is an error detection code that most data conlmunication networks use.

(68)is an interior gateway protocol that uses a distance vector algorithm to propagate routing information.

(69)is a transfer mode in which all types of information are organized into fixed form. cells on an asynchronous or nonperiodic basis over a range of media.

(70)is an identifier of a web page.