Tool use was once thought to be a uniquely human trait, easily seen in our daily lives. Because this behavior is present but less _______ in the animal kingdom, biologists must often set up specific puzzles to observe crows or chimps utilizing sticks to obtain food.
Which choice completes the text with the most logical and precise word or phrase?
recognizable
intriguing
significant
useful
Correct Answer: A
Choice A is the best answer. The text contrasts human tool use (easily seen) with animal tool use. Therefore, in animals, the behavior is less "recognizable" (able to be identified from observation) without specific puzzles.
Choice B is incorrect because the behavior is still "intriguing" (interesting) to researchers, hence the studies. Choice C is incorrect because the behavior is "significant" enough to be studied. Choice D is incorrect because the behavior's usefulness is not the point of comparison; the ease of observation is.