Which of the following sentences is correctly punctuated?

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1.The students: a majority compared to the college faculty, agreed to sign the contract.

2.The students—a majority compared to the college faculty—agreed to sign the contract.

3.The students, a majority compared to the college faculty—agreed to sign the contract.

4.The students; a majority compared to the college faculty—agreed to sign the contract.

Sentence 2 is correctly punctuated.

"The students—a majority compared to the college faculty—agreed to sign the contract."

Which sentence is punctuated correctly?

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1.I had to replace my headphones, which I bought less than a year ago.

2.I had to replace my headphones which I bought less than a year ago.

3.I had to replace my headphones which I bought, less than a year ago.

4.I had to replace my headphones which, I bought less than a year ago.

Sentence 1 is punctuated correctly.

"I had to replace my headphones, which I bought less than a year ago."