GMAT Sentence Correction (SC) | Re: Once each vote and piece of paper are burned

The subject should be SINGULAR since 'each' appears before the subject, hence IS should be used.
Option B is correct.

Once each vote and piece of paper are burned, the cardinals in the conclave are given time to reflect before casting their votes.
(A) Once each vote and piece of paper are burned
(B) Once each vote and piece of paper is burned
(C) Once each vote and piece of paper is burning
(D) Once each vote and pieces of paper are burned
(E) Once each votes and pieces of paper are burned

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