The reducing agent in the given equations

$$4 \mathrm{Ag}(s)+8 \mathrm{CN}^{-}(a q)+2 \mathrm{H}_2 \mathrm{O}(a q)+\mathrm{O}_2(g) \longrightarrow 4\left[\mathrm{Ag}(\mathrm{CN})_2\right]^{-}(a q)+4 \mathrm{OH}^{-}(a q)$$

$$2\left[\mathrm{Ag}(\mathrm{CN})_2\right]^{-}(a q)+\mathrm{Zn}(s) \longrightarrow {\left[\mathrm{Zn}(\mathrm{CN})_4\right]^{2-}(a q)+2 \mathrm{Ag}(s)}$$

$$a \mathrm{MnO}_4^{-}+b \mathrm{~S}_2 \mathrm{O}_3^{2-}+\mathrm{H}_2 \mathrm{O} \longrightarrow x \mathrm{MnO}_2 +y \mathrm{SO}_4^{2-}+z \mathrm{OH}^{-}$$

$$a$$ and $$y$$ respectively are

In which of the following compounds, an element exhibits two different oxidation states?

All Cu(II) halides are known, except the iodide, the reason for it is that