[单选题]
94.11: The critical angle of a material is the angl mwuybe9;zjsjk11f 4.e of 7srfjb :ng; wu yc55iuwk8y;-jr,8eg incidence for which the angle of refraction is8b,5j g7 r:8c;r s ueuyy;5 w-wgfkjin:
A. $0^{\circ}$
B. $30^{\circ}$
C. $45^{\circ}$
D. $90^{\circ}$
E. $180^{\circ}$
