If $\tau: F \rightarrow G$ is a natural transformation between additive functors, prove that $\tau$ gives chain maps $\tau_{\mathbf{C}}: F \mathbf{C} \rightarrow G \mathbf{C}$ for every complex C. If $\tau$ is a natural isomorphism, prove that $F \mathbf{C} \cong G \mathbf{C}$.