Which of the following sets could NOT be the lengths of the external diagonals of a right rectangular prism [a "box"]? (An external diagonal is a diagonal of one of the rectangular faces of the box.)<spanclass=′latex−bold′>(A)</span>{4,5,6}<spanclass=′latex−bold′>(B)</span>{4,5,7}<spanclass=′latex−bold′>(C)</span>{4,6,7}<spanclass=′latex−bold′>(D)</span>{5,6,7}<spanclass=′latex−bold′>(E)</span>{5,7,8}