If a blackout will occur for a long period of time, what is then needed to let the server functions uninterruptedly?

A.
Two UPS’s to keep the power going.

B.
A backup generator.

C.
You need a more expensive UPS devices are called online interactives.

D.
You need a Standby UPS device.

Explanation:
When the power goes out for extended periods of time, a backup generator is needed to keep power supplied to the system. Backup generators are normally powered by diesel fuel and they normally work together with the UPS, because they take time to start up and come online.
References:
Charles J. Brooks, Server+ Certification Exam Cram 2 (Exam SK0-002), QUE Publishing, Indianapolis, 2006, pp. 227-240.