Explanation
Correct answer: A.
Choice A is the best answer because it most logically completes the text’s discussion of microplastics as a proposed stratigraphic marker for the onset of the Anthropocene. The text indicates that although primary microplastics (used for industrial and other purposes) were invented and mass-produced beginning only around the mid-twentieth century, researchers found microplastics throughout European lake sediment profiles dating from the late eighteenth century to the present. According to the text, microplastics were in all layers of sediment likely because microplastics of certain shapes can rapidly migrate downward—that is, they can move into sedimentary layers that predate their manufacture. Since it can’t be assumed that the date of a layer in which microplastics are found indicates when the microplastics were first deposited, it follows that the lowest sedimentary layer containing microplastics can’t be treated as indicative of the chronological beginning of the Anthropocene.
Why the other choices are wrong
Choice B
Choice B is incorrect. The text explains that microplastics appeared in layers of sediment long predating 1950 likely because certain shapes allowed them to rapidly migrate downward—that is, the particles physically moved into older layers. This finding doesn’t cast doubt on the appropriateness of considering 1950 as the onset of the Anthropocene; it doesn’t suggest a flaw in thinking that humans’ impact on Earth shifted around 1950 but simply suggests that the position of microplastics in sediment isn’t useful as an indicator of such a shift.
Choice C
Choice C is incorrect because the issue presented in the text isn’t a lack of prevalence of microplastics (and though no quantity is indicated, microplastics were found across all the layers) but rather the inability to determine when microplastics were originally deposited, since the particles can migrate rapidly into older sediment layers.
Choice D
Choice D is incorrect because the text indicates that the likely cause of microplastics being present in all layers of the sediment profiles is a property of microplastic particles themselves (certain shapes allowing them to rapidly move downward into older layers), not a characteristic of European lake sediment or anything else specific to Europe. There’s no reason to think the issue of particle migration wouldn’t occur in other locations.