Question

Which of the following is not a prime number?

• A. $11$
• B. $21$
• C. $31$
• D. $41$

Answer 1

The correct option is B

$21$

a number that can be divided exactly only by itself and 1 is known as prime numbers.

$$\begin{gathered} 11=1\times 11 \\ 21=1\times 21=3\times 7 \\ 31=1\times 31 \\ 41=1\times 41 \end{gathered}$$

among the given numbers $21$ is the only numbers which has more than two factors.

Hence, $21$ is not a prime number.