Maybe mysterious. I hope that helps!
The correct answer is <span>3. The path through the park had been worn smooth by countless visitors: by tribes of passing children, by families with picnic baskets, by hikers with their dogs.
In the example above, you need to use a colon to separate these two parts of the sentence. The reason why you should use a colon instead of a comma is because you are listing these 'countless visitors,' including children, families, and hikers. So you are creating a list of people and animals who visit the park, and in order to do that, you ought to use a colon instead of a comma.
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