prep老，一套74. 各位指教！

从别人那里找来的，希望可以帮到你：
Your first thought here probably was - and should be - parallelism. However, you're right that 'are not ... but are' and 'are not ... but' would both be legitimately parallel constructions in this context. So, the reasoning goes, we should take the one with one fewer word. Right?
Here's the problem: In the context of this longer, more complicated sentence, the 'but' (without 'are' after it) is, unfortunately, ambiguous. Consider this sentence:
Jimmy was not the typical class president, loved by most of the students, but also hated by a fair number as well; rather, he was loved by all the students.
The most probable reading here is that all of the words 'loved by most of the students, but also hated by a fair number as well' are a description of 'the typical class president.'
If you look at choice C, it can be read this way; one could (probably should, even) take ALL the words '...favored because they enhance reproduction or survival, but simply random byproducts...' as an elaboration on the phrase 'products of natural selection'.