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

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

A.
Each data block in the database always corresponds to one operating system block.

B.
A data block is the smallest unit of input/output (I/O) in data files.

C.
Multiple tablespaces can share a single data file.

D.
Each segment contains one or more extents.

E.
It is possible to have tablespaces of different block sizes in a database.