If the following system has non-trivial solution,
- $px + qy + rz = 0$
- $qx + ry + pz = 0$
- $rx + py + qz = 0$,
then which one of the following options is TRUE?
- $p - q + r = 0 \text{ or } p = q = -r$
- $p + q - r = 0 \text{ or } p = -q = r$
- $p + q + r = 0 \text{ or } p = q = r$
- $p - q + r = 0 \text{ or } p = -q = -r$