For $a, b \in\{0,1, \cdots, 9\}$, a two digit number $a b$ is defined for $10 a+b$. A two digit number $a b$ is called a special prime if both $a b$ and $b a$ are primes. Then the total number of special primes is

• A

  $8$
• B

  $7$
• C

  $6$
• Correct answer
  D

  $9$

Hence, the correct answer is option: 'D'.