{"knowledge_schema": {"broad_category": "Crystallography \u2192 Symmetry and Optical Properties", "refinement": "This problem pertains to the classification of crystal systems based on their symmetry properties and their ability to exhibit optical activity.", "specific_scope": "The focus is on identifying achiral and non-polar crystal classes that can show optical activity, which is a property related to the interaction of light with the crystal structure.", "goal": "Determine which of the given crystal classes can exhibit optical activity despite being achiral and non-polar."}, "summary": "In crystallography, certain achiral and non-polar crystal classes can still exhibit optical activity due to their unique symmetry properties. The problem requires identifying such classes from a list of candidates. The answer is -4, indicating that this specific class possesses the necessary characteristics to show optical activity while adhering to the constraints of being achiral and non-polar."}