View the Exhibit and examine the command used to create the ZONEDATA table. 
The table contains a million rows for zonewise analysis in the DSS system. DML operations are performed very rarely on the table. You decide to prepare an index on the ZONE column to enhance the performance of the queries on the ZONE column.
Which type of index would you select in this scenario?
A.Bitmap index
B.Reverse key index
C.Normal B-Tree index
D.Function-based index
zone 的值只有四个 ，基数少，故用位图索引较好。
Bitmap index 的特点:
1. 对于大数据量的查询,bitmap index 能更有效的减少响应时间
2. 减少 index 的占用空间
索引相关 http://www.cnblogs.com/yumiko/p/5888732.html