This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
Cusp reduction can be considered in all of the following situations except: 
Select and explain. 
A. When the outline form has extended one third the distance from a primary groove to a cusp tip, B. When the outline form has extended two third the distance from an adjacent primary groove to the cusp tip, C. When the operator judges that adequate cuspal strength (adequate dentin support) remains, D. If the reduction of cusp improves the accessibility and visibility for operator
A: When the outline form has extended one third the distance from a primary groove to a cusp tip
Answer: A: When the outline form has extended one third the distance from a primary groove to a cusp tip 
Explanation: The most important aspect in the evaluation of a suspicious cusp is the judgment of the amount of remaining dentin support. Cusp reduction should be considered when the outline form has extended half the distance from a primary groove to a cusp tip. Cusp reduction is strongly recommended when the outline form has extended two third the distance from an adjacent primary groove to the cusp tip.
The exception to reducing a cusp, where an extension has been two third from a primary groove toward the cusp tip, is when the operator judges that adequate cuspal strength (adequate dentin support) remains. Reduction of cusps occurs as early as possible in the preparation process so as to improve access and visibility for the operator.