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\begin{document}
\begin{center}
{\bf \Large \bf CSC240 Winter 2021 Midterm Assessment Question 6}\\
YOUR NAME and STUDENT NUMBER
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\begin{enumerate}
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\item
\begin{question}
(5 marks)
Suppose that, for all $i \in \nats$, $A_i$ is a nonempty countable set.\\
Prove that $\cup\{A_i\ |\ i \in \nats\}$ is countable.
\end{question}
\begin{solution}
{\bf Solution}:
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