在学生关系中，对属性“性别”规定只能为“男”或“女”的这种约束属于(34)规则。

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Tile　hardware　are　the　parts　of　computer　itself　including　the(71)(CPU)and　related　microchips　and　micro　-circuitry,　keyboards,　monitors,　case　and(72)(including　floppy,　hard,　CD,　DVD,　optical,　tape,　etc...).　Other　extra　parts　called(73)or　devices　include　mouse,　printers,(74),　scanners,　digital　cameras　and　cards(sound,　colour,　video)etc...　Together　they　are　often　referred　to　as　a(75)or　PCs.

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C　is　sometimes　referred　to　as　a　'high　-level(66)language'.　Some　people　think　that's　an　insult,　but　it's　actually　a　deliberate　and　significant　aspect　of　the　language.　If　you　have(67)in　assembly　language,　youll　probably　find　C　very　natural　and　comfortable(although　if　you　continue　to　focus　too　heavily　on　machine　-　level　details,　youll　probably　end　up　with　unnecessarily　non-　portable　programs).　If　you　havent　programmed　in　assembly　language,　you　may　be　frustrated　by　C　's　lack　of　certain　higher　-　level　features.　In　either　case,　you　should　understand　why　C　was　de-　signed　this　way:　so　that　seemingly　-　simple　constructions　expressed　in　C　would　not　expand　to　arbitrarily　expensive(　in　time　or(68))machine　language　constructions　when(69).　If　you　write　a　C　program　simply　and　succinctly,　it　is　likely　to　result　in　a　succinct,　efficient　machine　language　executable.　If　you　find　that　the(70)resulting　from　a　C　program　is　not　efficient,　it's　probably　because　of　something　silly　you　did,　not　because　of　something　the　compiler　did　behind　your　back　which　you　have　no　control　over.　In　any　case,　there's　no　point　in　complaining　about　C's　low　-　level　flavor:　C　is　what　it　is.