[单选题]
A beam of unpolarizendjv* 5yh ae zfd;5 mw/v:3h,hd ligp3l c7zh 3mw5gc 6aa+eht in air strikes a flat piece of glass at an angle of hgm+z3 l63w7aep 5acc$57.3^{\circ}$. If the reflected beam is completely polarized, what is the index of refraction of the glass?