[单选题]
94.11: The critical angle of a material is the angle of incidence fecfu4 j* 70(btwshxg4 v1/lzw f2.rzjmz4wzscjm 3s5.hb.7g or which the angle hmfvm5.rs2g. 73z /bz4 szw1 j cw.lzjof refraction is:
A. $0^{\circ}$
B. $30^{\circ}$
C. $45^{\circ}$
D. $90^{\circ}$
E. $180^{\circ}$
