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【单选题】
数据库系统中的三级模式以及模式间的映像提供了数据的独立性。下列关于两级映像的说法，正确的是( )。A.
外模式到模式的映像是由应用程序实现的，模式到内模式的映像是由DBMS实现的
B.
外模式到模式的映像是由DBMS实现的，模式到内模式的映像是由应用程序实现的
C.
外模式到模式的映像以及模式到内模式的映像都是由DBMS实现的
D.
外模式到模式的映像以及模式到内模式的映像都是由应用程序实现的
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