[单选题]
A beam of unpolarized light in air strikeg yt0 89nrvlah9,qddau 1)wc-s a flat pieceufq+- k3sx0s3q .( kzjq9(cw p kae3qq of glass at an angle of skj-aqqkq3eqw.zf(0k cq+ p33xu9s($57.3^{\circ}$. If the reflected beam is completely polarized, what is the index of refraction of the glass?