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Verbal·Standard English Conventions·Boundaries

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Helical        widely understood to confer stability and efficiency in the locomotion of a variety of microscopic organisms—including bacteria, eukaryotic algae, and ciliates—bestows similar advantages, albeit via different propulsive modes, to larger oceanic macroplanktons, such as salps.

Which choice completes the text so that it conforms to the conventions of Standard English?

A

swimming is

B

swimming, is

C

swimming has been

D

swimming,