48.Which two statements are true regarding indexes? (Choose two.)

48.Which two statements are true regarding indexes? (Choose two.)
A.They can be created on tables and clusters.
B.They can be created on tables and simple views.
C.You can create only one index by using the same columns.
D.You can create more than one index by using the same columns if you specify distinctly different combinations of the columns.
答案：AD
A：正确
Introduction to Indexes
An index is an optional structure, associated with a table or table cluster, that can sometimes speed data access. 
Indexes are schema objects that are logically and physically independent of the data in the objects with which they are associated. 
Thus, you can drop or create an index without physically affecting the indexed table.
--这个是oracle的帮助文档中的描述
B：错误
C：错误，可以在一个列上创建多个索引，前提是需要和其他列进行组合
D：正确