Two distinct regular tetrahedra have all their vertices among the vertices of the same unit cube. What is the volume of the region formed by the intersection of the tetrahedra?<spanclass=′latex−bold′>(A)</span>121<spanclass=′latex−bold′>(B)</span>122<spanclass=′latex−bold′>(C)</span>123<spanclass=′latex−bold′>(D)</span>61<spanclass=′latex−bold′>(E)</span>62