According to the following diagram, A reduces  ${\mathrm{BO}}_2$ when the temperature is :

 

The Curves  Intersect at T = 1400°C

$$\begin{gathered} {\text{T<1400}}^{\text{0}}\text{C} \\ {\text{B+AO}}_{\text{2}}\to {\text{A+BO}}_{\text{2}}\text{\hspace{0.17em}\hspace{0.17em}will be\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}spontaneous} \\ {\text{T>1400}}^{\text{0}}\text{C} \\ {\text{A+BO}}_{\text{2}}\to {\text{B+AO}}_{\text{2}}\text{\hspace{0.17em}\hspace{0.17em}will\hspace{0.17em}be\hspace{0.17em}\hspace{0.17em}spontaneous} \end{gathered}$$