For a blackout, only the server is connected to a small generator. For the generator to produce enough electricity how long must it run?

A.
20 minutes or longer.

B.
5 to 10 seconds.

C.
10 minutes or longer.

D.
For the longest is about 25 minutes.

Explanation:
Small generators take from 5 to 10 seconds to come up, provided they are supplying power only to the servers.
References:
Charles J. Brooks, Server+ Certification Exam Cram 2 (Exam SK0-002), QUE Publishing, Indianapolis, 2006, pp. 227-240.