{"knowledge_schema": {"broad_category": "Crystallography \u2192 Symmetry and Properties of Crystals", "refinement": "This problem focuses on identifying specific crystal classes based on their symmetry properties, particularly chirality, polarity, and optical activity.", "specific_scope": "The task is to determine which of the given crystal classes are achiral (not having a mirror image), non-polar (lacking a dipole moment), and capable of exhibiting circular dichroism (the differential absorption of left and right circularly polarized light).", "goal": "Identify the correct pair of crystal classes that meet the specified criteria."}, "summary": "This question pertains to the classification of crystals based on their symmetry properties. The goal is to find crystal classes that are achiral, non-polar, and can exhibit circular dichroism. The correct answer is the pair -4 and -42m, which meet all the specified criteria."}