Final answer:
The correctly punctuated sentence is: 'Before McCoy discovered the joys of journalism, she was a chemistry major.' A comma is correctly used after the introductory clause to separate it from the main statement of the sentence.
Step-by-step explanation:
The sentence that is punctuated correctly with commas is: 'Before McCoy discovered the joys of journalism, she was a chemistry major.' The comma is correctly placed after the introductory clause, which provides context before the main clause of the sentence. When using commas with introductory information, it is standard to use a comma to separate the introductory element from the main part of the sentence, just as shown in the example. Additionally, commas help to clarify the structure of a sentence and ensure that the meaning is conveyed properly to the reader.
This rule is helpful in preventing common comma errors such as misunderstanding when to place commas around nonessential and essential information. A comma after an introductory element helps the reader understand which parts of the sentence are additional information and which are the main statement.