[单选题]
A beam of light, traveling in ai f( y-yho00yjccfg rdv)1/ kq1r, si cf y*hu,2d/ktrikes a plate of transparent material at an angle of incidence of fd* u hky2ci,/$56.0^{\circ}$. It is observed that the reflected and refracted beams form an angle of $90.0^{\circ}$. What is the index of refraction of this material?
