How many two digit prime numbers are there in which both digits are prime numbers? (For
example, 23 is one of these numbers but 31 is not, since 1 is not a prime number.)	Select.
A) 3 B) 4 C) 5 D) 8 E) 15
The second digit can only be 3 or 7, so the choice quickly narrows down to 23, 27,   33, 37, 53, 57, 73, and 77. Of these, 27, 33, and 57 are divisible by 3, and 77 by 7, leaving 23,   37, 53, and 73. It is easy to see that none of these is divisible by 2, 3, 5, or 7, and there is no   need to look at greater prime divisors since   root77 &lt; 11.   correct answer 4
<result>B</result>
How many two digit prime numbers are there in which both digits are prime numbers? (For
example, 23 is one of these numbers but 31 is not, since 1 is not a prime number.)