Which of the following sentences is correctly punctuated?

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

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

Which sentence is punctuated correctly?

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

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

What type of error can be found in the paragraph below?

Summer vacation comes only once a year, so my friends, and I plan to make the best of it. We're going to sleep until noon, eat all of the pizza and ice cream we can and then lay around the house playing video games. If we are feeling truly adventurous, my friends and I might even go outside for a few hours.
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subject-verb agreement
punctuation
spelling
capitalization

punctuation