$3\times6=18$, $3+6=9$  $2\times9=18$, $9+2=11$  Thus, the two digits of the number is $9$ and $2$.  The difference is $9-2=7$. 
There is a two-digit number. The product of its digits is $$18$$. The sum of the digits is $$11$$. The difference of the digits of this number is~\uline{~~~~~~~~~~}~. 
Options:
a. $$17$$ 
b. $$3$$ 
c. $$2$$ 
d. $$7$$ 
e. $$9$$ 