问题：

[单选]

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