单项选择题
The（）schemeinadatabasesystemisresponsibleforthedetectionoffailuresandfortherestorationofthedatabasetoastatethatexistedbeforetheoccurrenceofthefailure.
A.query B.test C.check D.recovery
单项选择题 （）statementcanperformacalculationandstoretheresultinavariablesothatitcanbeusedlater.
单项选择题 Dataitemsareaddedordeletedfromthelistonlyatthetopofthe（）.
单项选择题 （）meansthataprogramwrittenforonecomputersystemcanbecompiledandrunonanothersystemwithlittleornomodification.
