Let `O` be the origin and let PQR be an arbitrary triangle. The point S is such that ` vec O Pdot vec O Q+ vec O Rdot vec O S= vec O Rdot vec O P+ vec O Qdot vec O S= vec O Q` .` vec O R+ vec O Pdot vec O S` Then the triangle PQ has S as its: circumcentre (b) orthocentre (c) incentre (d) centroid