A user reports that after configuring the BIOS to boot to the hard drive first, the system still tries to boot from the optical drive. Which of the following is the MOST likely cause?

The optical drive itself would not cause the boot order to change; it is simply being booted from because the BIOS defaulted to it.

A power supply issue would cause the system to fail to start or behave erratically, not specifically cause boot order resets.

The CMOS battery provides continuous power to the CMOS chip that stores BIOS settings while the system is off. When the battery fails, all BIOS settings including the configured boot order are lost on each power cycle and revert to manufacturer defaults, which commonly list the optical drive before the hard drive. Replacing the CMOS battery and re-saving the boot order resolves the issue permanently.

A CD-ROM disk left in the drive could cause an attempted optical boot, but the question specifies no disk is present and the symptom is a persistent BIOS setting reset.