Let $A_1, A_2, \ldots$ be events in some sample space $\Omega$. Use Axiom Set 2.2.1 and the results of Exercise 6 to prove that
(a) $A_1 \cap A_2 \cap \cdots \cap A_N$ is an event for each positive integer $N$.
(b) $\bigcap_{n=1}^{\infty} A_n$ is an event.