21. A system of three polarizing sheets lies in the path of initially unpolarized light, which is in the z direction. The polarizing direction of the first sheet is parallel to the y axis, that of the second sheet is at an angle of 45° from the y axis, and that of the third sheet is parallel to the x axis. What fraction of the initial intensity of the light emerges from the three-sheet system?
(A) 1/2
(B) 1/4
(C) 1/8
(D) 1/16.