a and b are two non-zero vectors such that the projection of a on b is 0. The angle between a and b is:

$\displaystyle \overrightarrow{\mathrm{a}}$ and $\displaystyle \overrightarrow{\mathrm{b}}$ are two non-zero vectors such that the projection of $\displaystyle \overrightarrow{\mathrm{a}}$ on $\displaystyle \overrightarrow{\mathrm{b}}$ is $\displaystyle 0$ . The angle between $\displaystyle \overrightarrow{\mathrm{a}}$ and $\displaystyle \overrightarrow{\mathrm{b}}$ is :