Mathematics

Problem

For non-zero vectors

\mathbf{p}

and

\mathbf{q}

, the equality

\mathbf{p}-\mathbf{q}=\mathbf{q}-\mathbf{p}

holds if and only if a)

\mathbf{p}=-\mathbf{q}

|\mathbf{p}|=|\mathbf{q}|

\mathbf{p}=\mathbf{q}

\mathbf{p}\cdot\mathbf{q}=0