单项选择题

A. The index is not used by the optimizer but is maintained during DML operations.
B. The index is not used by the optimizer and is not maintained during DML operations.
C. The index is used by the optimizer only if a hint is specified in the query statement and is maintained during DML operations.
D. The index is used by the optimizer only if a hint is specified in the query statement but is not maintained during DML operations.