Cavosurface angle for amalgam is:: (A) >90°, (B) < 90°, (C) 90°, (D) 0°
Answer is C. The design of the cavosurface angle depends on the restorative material being used. Because of the low edge strength of amalgam and glass-ceramic, a 90-degree cavosurface angle produces maximal strength for these materials.
Sturdevants operative dentistry 7th edition page 132.