Which two statements are true about a bitmap index？ （）
A. It is recommended for the columns that have unique values.
B. It can be converted to a B-tree index by using the ALTER INDEX command.
C. It has a bitmap segment for each distinct value in the key column， containing a string of bits in which each bit represents the presence or absence of a key column value.
D. Updating the key column locks the whole bitmap segment that contains the bit for the key value to be updated.
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