Known as Earth’s “living skin,” biocrusts are thin layers of soil held together by surface-dwelling microorganisms such as
fungi, lichens, and cyanobacteria. Fortifying soil in arid ecosystems against erosion, ______
a recent study’s estimate is that these crusts reduce global dust emissions by 60 percent each year.
an estimated 60 percent reduction in global dust emissions each year is due to these crusts, according to a recent study.
these crusts reduce global dust emissions by an estimated 60 percent each year, according to a recent study.
a recent study has estimated that these crusts reduce global dust emissions by 60 percent each year.
Choice C is the best answer. The subject of the modiﬁer "fortifying soil in arid ecosystems against erosion" is "biocrusts."
Subject-modiﬁer placement requires a modiﬁer and its subject to be next to each other, so "biocrusts" or some variant
meaning "biocrusts" (in this case, "these crusts") must begin the missing clause.
Choice A is incorrect. Modiﬁers and their subjects must go next to each other. The subject of the modiﬁer "fortifying soil in
arid ecosystems against erosion" is "biocrusts," not "a recent study’s estimate." Choice B is incorrect. Modiﬁers and their
subjects must go next to each other. The subject of the modiﬁer "fortifying soil in arid ecosystems against erosion" is
"biocrusts," not "an estimated 60 percent reduction." Choice D is incorrect. Modiﬁers and their subjects must go next to each
other. The subject of the modiﬁer "fortifying soil in arid ecosystems against erosion" is "biocrusts," not "a recent study."