$$11$$，$$13$$，$$17$$，$$31$$，$$37$$，$$71$$，$$73$$，$$79$$，$$97$$． 
A two-digit prime number is still prime when the digits of its first and tenth digits are exchanged. There are  such prime numbers. 
Options:
a) $$6$$ 
b) $$7$$ 
c) $$8$$ 
d) $$9$$ 