A user reports that their system is starting to run slower, especially when opening large local files. The system has sufficient RAM for the users applications, but the hard drive seems to have higher than normal activity when opening large files. Which of the following would be the BEST tool to diagnose and troubleshoot this problem? （）

• A、Task Manager
• B、NTBACKUP
• C、FDISK
• D、Disk Defragmenter