In which of the following reactions, $MgO$ is not formed?

Question

$$\begin {array} {1 1} (a)\;Mg+CO_2 \longrightarrow & \quad (b)\;Mg+dil. HNO_3 \longrightarrow \\ (c)\;Mg+NO_2 \underrightarrow {\Delta} & \quad (d)\;Mg+B_2O_3 \longrightarrow \end {array}$$