A line of symmetry of a figure is a line along which the figure may be folded such that the two parts match up exactly.
The dotted line is a line of symmetry of the figure.
The dotted line is not a line of symmetry of the figure.
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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.