Intersection of infinite sets -question

Question

I am stuck at B . could any one help me ?

Answer 1

Let $n\in \bigcap_{k=1}^{\infty}A_k$ such that $n\geq1$. Then $n\in A_k$ for all $k\geq 1$. That implies $n$ is divisible by every $k\geq 1$, which is not possible. So $\bigcap_{k=1}^{\infty}A_k=\{0\}=A_0$.