Given $\vec{P} + \vec{Q} = \vec{P} - \vec{Q}$. Under what condition is this true?

• A
  $\vec{P} = \vec{Q}$
• B
  $\vec{Q}$ is a null vector.
• C
  $\vec{P}$ or $\vec{Q}$ is a null vector.
• D
  $\vec{P}$ is perpendicular to $\vec{Q}$.