Reptiles, by drawing their body heat directly from the Sun rather than

It's a logic thing.

What are you comparing? - how the reptiles obtain body heat
Do they "draw their body heat directly from the sun" or do they "burn calories to generate it"? We are selecting the action they perform to obtain body heat.

Since in the non underlined part we have ... by "drawing their body heat ... ," then to be parallel the underlined part should have "burning calories ..."

To be parallel, option (B) could be:
...by drawing their body heat directly from the Sun rather than generating body heat by burning calories...

Remember, the parallel elements should work standalone in the sentence.

Option (A)
Reptiles, by drawing their body heat directly from the Sun, can survive...
Reptiles, by burning calories to generate body heat, can survive...
Both correct.

Option (B)
Reptiles, by drawing their body heat directly from the Sun, can survive... - Correct
Reptiles, by the generation of body heat by burning calories, can survive... - Doesn't work. Who generates?

Most people would find this question hard. The absence of "by" after "rather than" in the original sentence is likely to especially confuse people.

Surprisingly, it is below 700 level!

Also, if this is an official question, request Bunuel to tag it accordingly.

This is why you have to figure out what the sentence is saying—at least on a basic level—before you start looking at isolated grammatical elements.

The sentence isn't about the Sun; it's about reptiles. If you look through the words and read them the same way you'd read a book, or an article, or the content of an e-mail (basically, just reading the words for what they say; NO thoughts of grammar yet at this point), you should be able to figure this out without a ton of trouble.

The comparison/substitution/parallelism in this case—linked by the phrase "rather than"—is between two contrasting ways of warming a body. Reptiles do one of these; they don't do the other one. (The second one describes how warm-blooded animals, such as humans, maintain body temperature.)
Only the first of these mechanisms involves the Sun, so the Sun is not going to be represented anywhere in the second one.





All of these questions are answered by the intended meaning of the sentence (= what the sentence 'WANTS' to say), which you need to figure out as described above—using a combination of common sense and given context.

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