单选题The ST_INFO master table has millions of rows that are updated very rarely. It has a STATE_CODE column that contains the value for 25 states. The table is frequently queried on the STATE_CODE column.  Which type of index would you suggest to improve th

The ST_INFO master table has millions of rows that are updated very rarely. It has a STATE_CODE column that contains the value for 25 states. The table is frequently queried on the STATE_CODE column.  Which type of index would you suggest to improve the query performance？（）