Which choice completes the text with the most logical and precise word or phrase? Some scientists have suggested that mammals in the Mesozoic era were not a very ______blank group, but paleontologist Zhe-Xi Luo's research suggests that early mammals living in the shadow of dinosaurs weren't all ground-dwelling insectivores. Fossils of various plant-eating mammals have been found in China, including species like Vilevolodon diplomylos, which Luo says could glide like a flying squirrel.

Choice C is the best answer because it most logically completes the text's discussion of the kinds of mammals alive during the Mesozoic era. As used in this context, 'diverse' means to have a significant amount of variety. The text indicates that some scientists have suggested that Mesozoic mammals can't be characterized in a certain way, then contrasts the view put forward by those scientists with Luo's research, which shows that Mesozoic mammals 'weren't all ground-dwelling insectivores' and instead were 'various.' This context suggests that some scientists have viewed Mesozoic mammals as being all alike, or not a very diverse group. Choice A is incorrect because it wouldn't make sense to say that some scientists have suggested that Mesozoic mammals weren't very 'predatory,' or that they didn't prey on other animals, since the text establishes a contrast between what some scientists have suggested and Luo's research showing that Mesozoic mammals 'weren't all ground-dwelling insectivores.' This context suggests that some scientists have regarded Mesozoic mammals as all being insectivores, or animals that prey on insects, not that some scientists have suggested that Mesozoic mammals didn't prey on other animals. Choice B is incorrect because it wouldn't make sense to say that some scientists have suggested that Mesozoic mammals weren't very 'obscure,' or concealed or not well known, since the text establishes a contrast between what some scientists have suggested and Luo's research showing that Mesozoic mammals were a varied group. There's no contrast between saying that the mammals weren't concealed or well known and the mammals being varied. Choice D is incorrect because it wouldn't make sense to say that some scientists have suggested that Mesozoic mammals weren't very 'localized,' or confined to a particular area, since the text establishes a contrast between what some scientists have suggested and Luo's research showing that Mesozoic mammals were a varied group. There's no contrast between saying that the mammals weren't localized and the mammals being varied. Although the text mentions mammal fossils found in China, nothing in the discussion of Luo's research addresses the limits of Mesozoic mammal habitats.