Another empty box

What is the most common use of the battery on a computer motherboard?

A) Provides additional power for the PCI Express slots

B) Maintains BIOS settings when power is not available

C) Provides additional power when the system is under heavy use

D) Boots the system when main power is not available

E) Switches between CPU clock rates

F) It’s the only place where I can guarantee the location of a battery in my house