Which two compounds demonstrate that transition elements can display variable oxidation states?
- A$\text{Cr}_2\text{O}_3$ and $\text{CrBr}_3$
- B$\text{CuSO}_4$ and $\text{CuCl}_2$
- C$\text{Fe}_2\text{O}_3$ and $\text{FeCl}_2$
- D$\text{NiO}$ and $\text{NiCl}_2$