Which three statements are true regarding the logical structure of the Oracle database？（）

• A、Each segment contains one or more extents.
• B、Multiple tablespaces can share single data file.
• C、A data block is the smallest unit of I/O for 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 OS block.