Add a comma, semicolon, or colon if needed. Otherwise, submit the text without any
additional punctuation.
Three of the most important breakthroughs in medicine were the identification of
insulin, the introduction of aspirin, and the discovery of penicillin.

Three of the most important breakthroughs in medicine were the identification of  insulin, the introduction of aspirin, and the discovery of penicillin.

Explanation:

The text is doesn't need any additional punctuation

(the answer is d)

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