Sentence Correction

Reptiles, by drawing their body heat directly from the Sun rather than burning calories to generate it, can survive on ten percent of the nourishment that a mammal of similar size would normally require

A) rather than burning calories to generate it
B) rather than the generation of body heat by burning calories
C) and not from generating it by burning calories
D) instead of by burning calories for generating it
E) instead of body heat generated by burning calories

OA is __A___.

I agree with the OA.
However, which gramatical unit is being made parallel?, the gerunds or the prepositional phrases?
Gerunds: drawing // burning.
Prepositional phrases: by drawing // (by) burning. - I have assumed that "by" has been omitted in the second prepositional phrase?, is that possible?

In the case of comparing just the gerunds that are part of prepositional phrases, is that possible in other parallel structures?