In which of the following half cells, electrochemical reaction is pH dependent?

The correct order of electron gain enthalpy ($$-$$ ve value) is :

$$\Delta$$G$$^\circ$$ vs T plot for the formation of MgO, involving reaction

2Mg + O₂ $$\to$$ 2MgO, will look like :

A $$\mathrel{\mathop{\kern0pt\longrightarrow} \limits_{}^{573\,K}} $$ Red phosphorus $$\mathrel{\mathop{\kern0pt\longrightarrow} \limits_{under\,pressure}^{heat\,;\,803\,K}} $$ B

Red phosphorus is obtained by heating "A" at 573 K, and can be converted to "B" by heating at 803 K under pressure.

A and B, respectively, are