Explanation:
A tablespace is a collection of one or more datafiles residing on your machine that Oracle treats as one logical area for storing data. This fact eliminates the choice that says one tablespace has only one datafile. Also, the two other choices basically state the same thing – that one datafile can contain many tablespaces – and this is just not true.
Reference:
Oracle 9i: Administrator’s Guide
Oracle 9i: Concepts

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