Choice A is the best answer. The author uses the phrase mainly to introduce a topic discussed at length in the second paragraph (lines 14–34)—namely, the growing connection Ethan sees himself forming with Mattie over the course of many evening walks during which they share similar feelings for the wonders of the natural world. In the context of the passage, the phrase evokes an image of two people walking eagerly and in harmony.

Choice B is not the best answer because while the phrase literally conveys Mattie’s attempts to keep up with Ethan’s pace, the phrase relates to times of leisure during which Ethan and Mattie walked arm-in-arm (see lines 1–4) rather than times of work. Moreover, the phrase is used primarily in a figurative way to suggest shared enthusiasm (see explanation for choice A).

Choice C is not the best answer because while the phrase literally describes Mattie’s attempts to keep up with Ethan’s pace, the context makes clear that Mattie and Ethan are not in competition with each other but rather enjoying times of leisure during which the two walk arm-in-arm (see lines 1–4). The phrase is instead used primarily in a figurative way to suggest shared enthusiasm (see explanation for choice A).

Choice D is not the best answer because while the phrase could in isolation be read as conveying some frustration on the part of Mattie, who had to expend extra effort to keep up with Ethan’s pace, the context makes clear that Mattie is not annoyed with Ethan but is instead enjoying times of leisure during which the two walk arm-in-arm (see lines 1–4). The phrase is instead used to suggest shared enthusiasm (see explanation for choice A).