The relationship between the subject and the predicate is pretty clear:
The subject is doing the action.
The predicate is what is being done and provides information about the subject.
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.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
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.