考虑下述文法，S为开始符号　　　　G1[S]：S→A　　　　　　　　　　　A→aAb　|　ab　　　　G2[S]　　S→AA→aA　|a|　　　　下列结论中为真的是(28)。

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Password　is　a(n)(71)series　of　characters　that　enables　a　user(72)a　file,　computer　or　program.　On　multi　-　user　systems,(73)user　must　enter　his　or　her　password(74)the　computer　　will　respond　to　commands.　The　password　helps　ensure　that　unauthorized　users　do　not　access　the　computer.　In　addition,　data　files　and　programs　may　require　a　password.

Ideally,　the　password　should　be　something(75)could　guess.　In　practice,　most　people　choose　a　password　that　is　easy　to　remember,　such　as　their　name　or　their　initials.　This　is　one　reason　it　is　relatively　easy　to　break　into　most　computer　system.

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The　program　memory　serves　basically　as　a　place(66)instructions,　the　coded　pieces　of　data(67)direct　the　activities　of　the　control　processing　unit(CPU).　A　group　of(68)related　in-　structions　stored　in　memory　is　referred　to　as　a　program.　The　CPU　"read"　each　instruction　from　memory　a　logically(69)sequence　and　"use"　it　to　initiate　program　action.　If　the　program　structure　is　coherent　and(70),　processing　procedures　produce　intelligible　and　useful　results.