Which of the following statements about the stability of oxidation states in aqueous solution are correct?

1. A$\mathrm{Fe}{}^{3+}$ is formed readily from $\mathrm{Fe}{}^{2+}$ because the third ionisation of Fe gives a $d^5$ ion and its third ionisation enthalpy is lower than that of Mn
2. B$\mathrm{Cr}{}^{3+}$ is unusually stable in water because of its $t_{2g}^3$ configuration in an octahedral field
3. C$\mathrm{Co}{}^{3+}$ is a strong oxidising agent in water ($E^{\circ }=+1.97\mathrm{V}$ for $\mathrm{Co}{}^{3+}/\mathrm{Co}{}^{2+}$)
4. D$\mathrm{Cu}{}^{+}$ is stable in water because it is $d^{10}$