An index called ORD_CUSTNAME_IX has been created on the CUSTNAME column in the ORDERS table using the following command：   SQL>CREATE INDEX ord_custname_ix ON orders（custname）；   The ORDERS table is frequently queried using the CUSTNAME column in the WHERE clause. You want to check the impact on the performance of the queries if the index is not available. You do not want the index to be dropped or rebuilt to perform this test.  Which is the most efficient method of performing this task？（）

• A、 disabling the index
• B、 making the index invisible
• C、 making the index unusable
• D、 using the MONITORING USAGE clause for the index