Consider the query:
Mysql＞ SET @run = 15;
Mysql＞ EXPLAIN SELECT objective, stage, COUNT (stage)
FROM iteminformation
WHERE run=@run AND objective=’7.1’
GROUP BY objective,stage
ORDER BY stage;

The iteminformation table has the following indexes;
Mysql＞ SHOW INDEXES FROM iteminformation:

This query is run several times in an application with different values in the WHERE clause in a growing data set.
What is the primary improvement that can be made for this scenario?（）

A.Do not pass a user variable in the WHERE clause because it limits the ability of the optimizer to use indexes
B.Add an index on the objective column so that is can be used in both the WHERE and GROUP BY operations
C.Drop the run_2 index because it has caused a conflict in the choice of key for this query
D.Execute the run_2 index because it has caused a conflict in the choice of key for this query
E.Add a composite index on (run,objective,stage) to allow the query to fully utilize an index