Explanation

Creating a filter: as we read the original sentence, we keep an eye on the placement of the word "also." But that placement could be correct, if the intended meaning is that the lower elevations, in addition to the upper elevations, have conditions favorable to the markor. We'll have to look for defects in the other choices and come back to (A).

Finding objective defects: choice (B) is defective in that "favorable" and "to" are separated by the word "conditions," violating the idiomatic usage of "favorable." Choice (B) is out. Choice (C) introduces a pronoun with no reference, "they," so it's out. Choice (D) has the same problem as (B), so it's out. Choice (E) has an idiomatic error, since "favorable for" is not proper usage. We are left with one choice. The correct answer is (A).