Question #N1099

The most common type of punctuation in English writing, the comma, is frequently used to separate items in a series. But when a series contains three or more items, the comma ______ the last item from the first item in the series, not the first item from the last item.

Which choice completes the text so that it conforms to the conventions of Standard English?
A. separates
B. separate
C. is separating
D. separated