21. Which three statements are true about logical structures of the Oracle database? (Choose three.)
A.Each segment contains one or more extents.
B.Multiple tablespaces can share a single data file.
C.A data block is the smallest unit of input/output (I/O) in data files.
D.It is possible to have tablespaces of different block sizes in a database.
E.Each data block in the database always corresponds to one operating system block.
Answer: ACD
答案解析：
参考存储体系结构：http://blog.csdn.net/rlhua/article/details/12233167
由下图可知：
A每个段包含一个或多个区，正确。
B答案：一个数据文件只能属于一个表空间，故错误。
C答案：Oracle data block是最小的I/O单位，正确
D答案：可以是不同的块大小，需要指名非标准块的位置。
E答案：每个数据库里面的数据库对应一个或多个操作系统块，故E错误。