Video Explanation

Text Explanation

Split #1a: parallelism in the comparison.  Fundamentally, we want to compare the situation of ancient hunters to the situation of modern long-distance backpackers.  These two groups must be presented in parallel.  Before the underlined section, the first group follows the preposition "for" ---- "for many ancient hunters" ---- so the preposition "for" must also appear before "modern long-distance backpackers."  Only choices (A) and (E) correctly repeat the word "for". 

Split #1b: what are we comparing?  We are not comparing just a single noun ---- [noun] of ancient hunters vs. [same noun] of modern long-distance backpackers.  If we were comparing a single noun, we could use "like."  Since we are not, (A) is incorrect.  The word “as” is a subordinating conjunction, which is followed by a full noun + verb clause, so it is used to compare events, actions. And this is the comparison that we see in the sentence. In the first clause, we have a very long subject ("Balancing the need for sufficient food supplies with what constitutes a manageable load to carry was undoubtedly a concern at times") followed by a verb phrase ("was undoubtedly a concern at times for many ancient hunters and gatherers"). Thus, we're going to need another full subject + verb clause to be parallel to the main clause. (A) & (B) construct the comparison so that it focuses on a noun, the pronoun “that”, so “like that” would be correct and “as that” would be incorrect.  Unfortunately, both of these are merely phrases, not the full subject + verb clause that would be parallel to the main clause.  Answers (C), (D), and (E) all have “as” with a subject & verb, so with respect to the “like” vs. “as” question, all three work.  Distinguishing among (C), (D), and (E) depends on the parallelism.  The verb in the main clause is “was”, a form of the verb “to be” — this balancing “was a concern” — and a proper parallel to a form of “to be” cannot be a form of “to do.”  Forms of the verb “to do” can form the proper parallel to almost any action verb, but not a form of “to be.”  Both (C) & (D) have “do”, which is incorrect parallelism. This leaves (E) as the correct option.

Choice (E) is the only possible answer.