A complete sentence expresses a complete thought. Also, a complete sentence always contains a subject and a predicate.
A subject (a noun or a pronoun with modifiers) tells who or what the sentence is about.
A predicate, such as a verb, tells what happens or what the subject is doing.
The subject is what the sentence is about, so it usually comes at the beginning of a sentence and is followed by the predicate.
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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.