ABO (American Board of Opticianry) Practice Test 2024 - Free ABO Practice Questions and Study Guide.

A combination of prism is considered a "canceling situation" if the directions of the base are opposite of each other. This creates a balance that cancels out the effects of the prism. A and B are both incorrect because they are combinations of prism that have the same direction of the base. This means they would add together, rather than cancel out. D is also incorrect because it is a combination of prism that have the same direction of the base. In this case, the effects would also add together. C is the only correct choice because it is a combination of prism that have the opposite direction of the base. This would not be a canceling situation, as the effects would not balance each other out. Instead, they would add together and create a stronger prism effect.