A sentence is a group of words that forms a complete thought. It has both a subject and a verb.
My friends walk along the path.
A run-on sentence is made up of two sentences that are joined without end punctuation or with just a comma (which is called a comma splice).
I knocked on the door it opened.
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The correct answer is
The smallest number is the one that comes first while counting.
To arrange the given numbers in order from smallest to greatest, find the smallest number among all the given numbers.
21 is the smallest number.