70-451 Question #51: Real Exam Question with Answer & Explanation

You have a table that contains 5 million rows. The table has a clustered index. You plan to add an additional index on a column that contains 80 percent null values. The column is used extensively in WHERE clauses. You need to implement an index strategy for the column. The solution must minimize the amount of storage space required for the index. Which type of index should you use?