You work in a company which is named Wiikigo Corp. The company uses SQL Server 2008. You are the administrator of the company database. Now you are in charge of a SQL Server 2008 instance. There is a database which is named Sellings in the instance. The Sellings database contains a table named Productions. The table is used to stores information about all types of products. The Productions table is often queried by users on the basis of the RadioSize column. The RadioSize column contains the NULL value for all products other than Radios. Currently no index exists on the RadionSize column. According to the requirement of the company CIO, you have to optimize the query performance and reduce the effect on the disk space to the least. So what action should you perform to achieve this goal?（）