"$$a$$ is a factor of $37$" means $37$ is divisible by $a$. 
Given $$a$$ is a factor of $37$, then ． 
Options:
(A) $a$ can only be $1$ 
(B) $a$ can only be $37$ 
(C) $a$ can be 1 or $37$ 
(D) $a$ can be any number 