Which of the following is not a prime number ?
$a.)\ 911 \ \ \ \ \ \ \ \ \ \ b.)\ 919 \\ \color{green}{c.)\ 943} \ \ \ \ \ \ \ \ \ \ d.)\ 947$
This was asked in my exam and the time given per question was $1-3\ \text{ min}$
If the time was $10\ \text{min}$ for this question I would have solved this comfortably but since the time was less i couldn't solve it with dividing every option with shorter primes .
I look for a simple and short way.
I have studied maths up to $12$th grade.