David is thinking of a two-digit number. He provides four statements about it, and exactly three of the following four statements are true:
(1) It is a multiple of 9.
(2) It is greater than 50.
(3) It is a prime number.
(4) The sum of its digits is 10.
This information is sufficient to uniquely determine the number. What is the tens digit of David's number?
3
5
6
7
9
