Find the square root of 441 to avoid an asteroid hitting the earth.

The correct option is B 21
$\sqrt{441}=\sqrt{3\times 3\times 7\times 7}$

$=\sqrt{3^2\times 7^2}$

$=\sqrt{{21}^2}(\because a^m\times b^m=(ab{)}^m)$

$=({21}^2{)}^{\frac{1}{2}}$

$={21}^{2\times \frac{1}{2}}(\because (a^m{)}^n=a^{mn})$

$=21$

Therefore, the square root of 441 is 21.