Standard reduction potentials are: $\mathrm{Cr}{}^{3+}/\mathrm{Cr}{}^{2+}=-0.41\mathrm{V}$, $\mathrm{Fe}{}^{3+}/\mathrm{Fe}{}^{2+}=+0.77\mathrm{V}$, $\mathrm{Mn}{}^{3+}/\mathrm{Mn}{}^{2+}=+1.57\mathrm{V}$, $\mathrm{Co}{}^{3+}/\mathrm{Co}{}^{2+}=+1.97\mathrm{V}$, $\mathrm{Cu}{}^{2+}/\mathrm{Cu}=+0.34\mathrm{V}$. Which of the following reactions are spontaneous under standard conditions in acidic aqueous solution?

1. A$\mathrm{Co}{}^{3+}+\mathrm{Mn}{}^{2+}\to \mathrm{Co}{}^{2+}+\mathrm{Mn}{}^{3+}$
2. B$\mathrm{Fe}{}^{3+}+\mathrm{Cr}{}^{2+}\to \mathrm{Fe}{}^{2+}+\mathrm{Cr}{}^{3+}$
3. C$\mathrm{Cu}{}^{2+}+2\mathrm{Fe}{}^{2+}\to \mathrm{Cu}+2\mathrm{Fe}{}^{3+}$
4. D$\mathrm{Mn}{}^{3+}+\mathrm{Fe}{}^{2+}\to \mathrm{Mn}{}^{2+}+\mathrm{Fe}{}^{3+}$