which index type is most likely to be used to improve the performance of some queries where the data is of low cardinality?

You will be implementing a data warehouse for one of your customers. In your design process,
which index type is most likely to be used to improve the performance of some queries where the
data is of low cardinality?

A.
Bitmap indexes

B.
B*-tree indexes

C.
Reverse indexes

D.
Invisible indexes

Explanation:
Bitmap indexes are a highly compressed index type that tends to be used primarily
for data warehouses.
Characteristic of Bitmap Indexes
* For columns with very few unique values (low cardinality)
* Columns that have low cardinality are good candidates (if the cardinality of a column is <= 0.1 %
that the column is ideal candidate, consider also 0.2% 1%)
* Tables that have no or little insert/update are good candidates (static data in warehouse)
* Stream of bits: each bit relates to a column value in a single row of table
Reference: The Secrets of Oracle Bitmap Indexes,

http://www.akadia.com/services/ora_bitmapped_index.html