Which performance counter should you monitor?

Your network contains a database server that runs Windows Server 2003 Standard Edition Service Pack 2 (SP2). The server has 1 GB of RAM and a single hard disk.
Queries against the databases on the server are very slow. You discover that the hard disk on the server is very active.
You are evaluating whether to add RAM to the server.
You need to use System Monitor to find out whether adding RAM will decrease disk activity.
Which performance counter should you monitor?

A.
LogicalDisk : % Free Space

B.
Memory : Pages Faults/sec

C.
Memory : Free System Page Table Entries

D.
Server Work Queues : Queue Length