Parallel, Perpendicular, Intersecting Lines
Lines that stay same distance apart are called parallel lines. They do not meet, howsoever far they are extended.
Lines that meet each other at right angles are called perpendicular lines.
Lines that meet each other at a point are called intersecting lines.
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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.