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.

English

PASSAGE: <div><p>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, <span aria-hidden="true">______</span><span class="sr-only"></span></p></div> QUESTION: Which choice completes the text so that it conforms to the conventions of Standard English?