Choice B is the best answer. The researchers described in Passage 1 “hid behind a blind” (Passage 1, line 16) to avoid being seen by the crow. The author of Passage 2, on the other hand, made no attempt to conceal his presence; in fact, as he describes it, he “led” the ravens in his study on “walks” (Passage 2, lines 1–2), during which he “touched specific objects” (Passage 2, line 4) and then watched to see whether the birds touched the same objects. The author of Passage 2 notes that the ravens “soon became more independent” (Passage 2, line 8), going their own way rather than continuing to follow the author. From this it is clear that the author of Passage 2, unlike the researchers described in Passage 1, intentionally made the birds aware of his presence.

Choice A is not the best answer because while a case could be made that the author of Passage 2 gave the ravens a problem to solve (Which new objects are best to touch?), the researchers described in Passage 1 presented the crows with a problem as well: how to extract meat from a log. Thus, presenting birds with a problem to solve was not a difference between the experiments.

Choice C is not the best answer because both the researchers described in Passage 1 and the author of Passage 2 consciously manipulated the birds’ surroundings. The crow researchers placed meat pieces in a log and a pandanus plant behind the log (see Passage 1, lines 14–16). The author of Passage 2 put unfamiliar objects on a path for the ravens to find (see Passage 2, lines 13–14). Thus, conscious manipulation of the birds’ surroundings was not a difference between the experiments.

Choice D is not the best answer because there is no evidence that the author of Passage 2 tested the ravens’ tool-using abilities. The passage instead indicates that the author recorded observations about the birds’ interactions with objects naturally occurring in and artificially introduced into the environment.