Pteropods are small swimming snails with thin, delie calcium carbonate shells. These animals are thought to

Choice D is the best answer because it most accurately describes the main purpose of the text, which is to present findings that suggest a previously held concern about the effects of ocean acidifiion on pteropod shells may be unwarranted. The text introduces the concern that pteropods are thought to be particularly vulnerable to ocean acidifiion, or lower pH levels, due to the susceptibility of their calcium carbonate shells to dissolution at lower pH levels. However, the text then describes findings from a recent study that suggest this concern may be unjustified, insofar as the protective periostracum coating on a pteropod’s shell prevents dissolution, and even when the coating is breached, a pteropod can rebuild the inner shell wall, reducing the damage.

Choice A is incorrect because the text doesn’t call for additional research or suggest that more research is needed on biological mechanisms that improve pteropod survival rates. Instead, the text discusses a recent study showing that pteropods may not be as vulnerable to ocean acidifiion as is feared. Choice B is incorrect because the study discussed in the text doesn’t address calcium carbonate’s role in protecting the periostracum of pteropods. According to the text, the study addresses the periostracum’s role in protecting pteropods’ calcium carbonate shells from dissolution due to ocean acidifiion. Choice C is incorrect because the text doesn’t address how ocean acidifiion has altered pteropod behavior over time. Instead, the text focuses on the potential effects of ocean acidifiion on pteropods’ shells and the mechanisms protecting against those effects.