Let $0 \rightarrow \mathbf{C}^{\prime} \stackrel{l}{\longrightarrow} \mathbf{C} \stackrel{p}{\longrightarrow} \mathbf{C}^{\prime \prime} \rightarrow 0$ be an exact sequence of complexes in which $\mathbf{C}^{\prime}$ and $\mathbf{C}^{\prime \prime}$ are acyclic; prove that $\mathbf{C}$ is also acyclic.