Subjects and verbs must agree in number. If the subject is singular, the verb must be singular; and if the subject is plural, the verb must be plural.
Sandra draws a picture. [singular subject takes singular verb]
Stacy and Ben argue all the time. [plural subject takes plural verb]
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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.
21,27,23
21 is the smallest number.