Greetings all!
I have a question regarding an answer given as correct on the SAT. I should say that I am asking here as an SAT instructor with some experience, but I can't make sense of the response. It truly seems incorrect. Am I missing something?
>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,
'D' is given as the correct reponse.
I can see that the first comma in answer 'D' sets off a nonessential clause. I see that within that clause we have another parenthetical enclosed within em dashes. After this, we reach the predicate verb. How is possible that the closing comma is omitted for the original nonessential clause?
Looking forward to your input and thanks in advance.