Which three statements are true regarding the logical structure of the Oracle database? (Choose three.)
A. It is possible to have tablespaces of different block sizes in a database.
B. A data block is the smallest unit of I/O for data files.
C. Multiple tablespaces can share single data file.
D. Each data block in the database always corresponds to one OS block.
E. Each segment contains one or more extents.
Answer: ABE
Oracle逻辑结构（TableSpace→Segment→Extent→Block） - X.Jan - 博客园
一个数据库是由多个表空间（tablespace）组成，一个表空间又由多个段（segment）组成，一个段又由多个区（extent）组成，一个区则由多个块（block）组成。
Specifying Database Block Sizes
Specifying Nonstandard Block Sizes for Tablespaces
You can create tablespaces with block sizes different from the standard database block size, which is specified by the DB_BLOCK_SIZEinitialization parameter. This feature lets you transport tablespaces with unlike block sizes between databases.
Use the BLOCKSIZE clause of the CREATE TABLESPACE statement to create a tablespace with a block size different from the database standard block size. In order for the BLOCKSIZE clause to succeed, you must have already set the DB_CACHE_SIZE and at least one DB_nK_CACHE_SIZE initialization parameter. Further, and the integer you specify in the BLOCKSIZE clause must correspond with the setting of one DB_nK_CACHE_SIZE parameter setting. Although redundant, specifying a BLOCKSIZE equal to the standard block size, as specified by the DB_BLOCK_SIZE initialization parameter, is allowed.
The following statement creates tablespace lmtbsb, but specifies a block size that differs from the standard database block size (as specified by the DB_BLOCK_SIZE initialization parameter):
CREATE TABLESPACE lmtbsb DATAFILE '/u02/oracle/data/lmtbsb01.dbf' SIZE 50M
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 128K
BLOCKSIZE 8K;