Let $R$ be a ring with identity, and let $I$ be a left $R$-module. Then $I$ is an injective left $R$-module if and only if, for every short exact sequence of left $R$-modules
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\begin{align*}
0 \longrightarrow A \xrightarrow{\iota} B \xrightarrow{\pi} C \longrightarrow 0,
\end{align*}