Which type of UPS (Uninterruptible Power Supply) can undergo maintenance while remaining online?

A.
hot-swap battery enabled UPS (Uninterruptible Power Supply)

B.
smart slot enabled UPS (Uninterruptible Power Supply)

C.
hot-spare battery enabled UPS (Uninterruptible Power Supply)

D.
extended runtime enabled UPS (Uninterruptible Power Supply)

E.
smart cable enabled UPS (Uninterruptible Power Supply)