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The following is a proof that every subring of a cyclic ring is a cyclic ring.
\begin{proof}
Let $R$ be a cyclic ring and $S$ be a subring of $R$. Then the additive group of $S$ is a subgroup of the additive group of $R$. By definition of cyclic group, the additive group of $R$ is \PMlinkname{cyclic}{CyclicGroup}. Thus, the additive group of $S$ is cyclic. It follows that $S$ is a cyclic ring.