List all the combinations of fourfour objects x comma y comma z comma and sx, y, z, and s taken twotwo at a time. Choose the correct answer below. A.xy comma xz comma xs comma yx comma yz comma ys comma zx comma zy comma zs comma sx comma sy comma sz nothing xy comma xz comma xs comma yx comma yz comma ys comma zx comma zy comma zs comma sx comma sy comma szxy, xz, xs, yx, yz, ys, zx, zy, zs, sx, sy, sz nothing B.x comma y comma z comma s x comma y comma z comma sx, y, z, s C.xy comma xz comma xs comma yz comma ys comma zs xy comma xz comma xs comma yz comma ys comma zsxy, xz, xs, yz, ys, zs D.xx comma xy comma xz comma xs comma yy comma yz comma ys comma zz comma zs comma ss nothing xx comma xy comma xz comma xs comma yy comma yz comma ys comma zz comma zs comma ssxx, xy, xz, xs, yy, yz, ys