Which one of the following describes a set of relationships that is most ... ...

Which one of the following describes a set of relationships that is most closely analogous to the relationships between plants and their primary and secondary substances?
(A) Electrical power for the operation of devices such as lights and medical instruments is essential to the proper functioning of hospitals; generators are often used in hospitals to provide electricity in case their usual source of power is temporarily unavailable.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in Dec 2012 LSAT (PT 68): Reading Comprehension