$$11$$，$$13$$，$$17$$，$$31$$，$$37$$，$$71$$，$$73$$，$$79$$，$$97$$． 
A two-digit prime number is still prime when the digits of its first and tenth digits are exchanged. There are  such prime numbers. 
1: $$6$$ 
2: $$7$$ 
3: $$8$$ 
4: $$9$$ 
5: $$10$$ 