Several queries are executed on the Manufacturing Company database. One query accesses the data about employees of a particular employee type and a particular department, another-about the employees of a particular employee type. Which index may improve performance of both queries? a. Composite index on emplType, deptCode. b. Composite index on rD, emplType, deptCode. c. Composite index on dept Code, emplType. d. Single-column index on emplType.