A researcher is culturing mouse cells in the laboratory. When he treats cells with four vitamins―W, X, Y, and Z―these cells survive and grow; he knows that some of these vitamins are essential nutrients. However, when he treats cells by removing one vitamin at a time, he gets the following results: vitamin W, cells live; vitamin X, cells live; vitamin Y, cells die; vitamin Z, cells die. Based on these results, which of the vitamins are likely essential nutrients?