Which of the following reactions is spontaneous? \bigcirc N i^{2+}+2 A g \rightarrow 2 A g^{+}+N i \bigcirc 2 I^{-}+A g \rightarrow A g^{+}+I_{2} \bigcirc C u^{2+}+P b \rightarrow P

b^{2+}+C u \mathrm{Co}^{2+}+\mathrm{Cu} \rightarrow \mathrm{Co}+\mathrm{Cu}^{2+}