Sentence Correction (SC) | Re: Reptiles, by drawing their body heat directly from the Sun rather than

I am having problem in arriving at the part of sentence which needs to be parallel. Like in option A : "By drawing their body heat from the Sun" is parallel to "Burning calories to generate it".
But why "The Sun" is not compared to "Burning calories to generate it". And as per this it is not parallel.
How to come to the conclusion that comparison is done with the entity just before comparison word(rather than) ---
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