Explanation
Correct answer: D.
Choice D is the best answer. The text describes a study about how reef-building corals capture microplastics from ocean waters, which found that these corals are storing 20 million kilograms of microplastics in their skeletons and tissues.
Why the other choices are wrong
Choice A
Choice A is incorrect. While the researchers are studying “reef-building corals,” the focus of their study isn’t how the corals build the reefs. Rather, they’re studying how corals may be storing microplastics from ocean waters.
Choice B
Choice B is incorrect. The text doesn’t mention any unanswered questions about the impact of corals on ocean ecosystems—rather, the study assesses one interaction between corals and microplastics.
Choice C
Choice C is incorrect. The text does say this, but it’s a detail—not the main idea. The main idea of the text is about the study that found that corals may be storing microplastics from ocean waters in their skeletons and tissues.