It is known that the positive whole number $$n$$ is divisible by $$21$$ and by $$9$$. Which of the answers below can be the number of divisors of the number $$n$$? ($$2002$$ Math kangaroo Problems, Level $$7-8$$, Question \#$$21$$) 
Overseas Competition->Knowledge Point->Number Theory Modules->Factors and Multiples->Theorem of the Number of Factors of a Number->Applying Theorem of the Number of Factors of a Number Directly->The Total Number of Factors
$n$ is divisible by $63$. $63=3^{2}\times7$. $63$ has $(2+1)\times(1+1)=6$ divisors. Thus, $n$ has at least $6$ divisors, and $n$ can\textquotesingle t have an odd number of divisors. Option $D$ is the only possible answer. 