Question 3
Let $Oxyz$ be a right trihedral angle, and let fixed points $A,B,C$ lie respectively on $Ox,Oy,Oz$. A variable sphere $(E)$ always passes through $A,B,C$ and meets the edges $Ox,Oy,Oz$ again at $A',B',C'$, respectively. Let $M,M'$ be the centroids of triangles $ABC,A'B'C'$, respectively. As $(E)$ varies, on what figure does the midpoint $S$ of $MM'$ move?