若关系中的某一属性组的值能唯一地标识一个元组，则称该属性组为(54)；若关系中的某一超码，当去掉其中任一属性后，均不再为超码，则称其为(55)。

(52)

(51)

(50)

Sometimes　the　processes　in　which　people　are　engaged　are　dependent　upon　other　processes　being　completed　first,　possibly　by　other　people.　For　example,　on　an　automobile(71),　it　is　necessary　　for　the　body　to　be　constructed　before　the　doors　can　be　added　on.　Attempting　to　add　the　doors　first　would　be　pointless,　because　there　would　be　nowhere　to　attach　them.　Thus,　if　there　is　no　body　ready　to　have　doors　added,　the　process　for　adding　doors　has　to　be(72)until　the　process　for　creating　the　next　body　is　complete.　This　is　known　as(73)

Obviously,　there　are　many　cases　where　people　have　to　wait　for　other　people　to　do　something.　However,　　if　everyone　in　such　a　situation　is　waiting　for　someone　else　to　do　something　first,　then　no　one　will　ever　be　able　to　do　anything.　When　discussing　processes,　this　situation　is　known　as(74),　because　all　of　the　involved　processes　are　effectively　dead　because　of　locks　they　have　on　resources.　Usually　someone　in　such　a　situation　will　eventually　notice　and　abort　the　process,　allowing　normal　flow　to(75),　and　often　warning　signs　can　even　be　spotted　ahead　of　time　to　avoid　'it　in　the　first　place.

(49)

(46)

(47)

(44)

(45)

(42)