ISL 1973 Problem 2

Let $OX, OY$ and $OZ$ be three rays in the space, and $G$ a point "between these rays" (i. e. in the interior of the part of the space bordered by the angles $Y OZ, ZOX$ and $XOY$ ). Consider a plane passing through $G$ and meeting the rays $OX, OY$ and $OZ$ in the points $A, B, C$ , respectively. There are infinitely many such planes; construct the one which minimizes the volume of the tetrahedron $OABC$ .