Given positive integer $ n > 1$ and define \[ S = \{1,2,\dots,n\}. \] Suppose \[ T = \{t \in S: \gcd(t,n) = 1\}. \] Let $ A$ be arbitrary non-empty subset of $ A$ such thar for all $ x,y \in A$, we have $ (xy\mod n) \in A$. Prove that the number of elements of $ A$ divides $ \phi(n)$. ($ \phi(n)$ is Euler-Phi function)