To form the present continuous, we add “ing” at the end of the main verb and we add the auxiliary verb to be - is, am, are before it:
subject + auxiliary verb + main verb
to be (is / am / are) base + ing
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.