Which three statements are true about logical structures of the Oracle database（）

• 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.