The upside-down A stands for anything. It is a symbol. Option a is correct. We can find such S sets, such as integer set Z, which satisfies any A, B belongs to Z, a+b belongs to Z, A-B belongs to Z, similarly, rational number set Q is also a harmonic set. A similar judgment option B is also correct. Take option C as an example. Two unequal harmonious sets: integer set Z and rational number set Q are subsets of real number set R, but the intersection of the two sets is not an empty set. Option D can find a counterexample, or these two harmonic sets: integer set Z and rational number set Q are subsets of real number set R, but the union of the two sets is not real number set R. Option C is really difficult to judge. Sorry. When doing multiple-choice questions, you don't have to do proof questions. The correct conclusion can be drawn by giving positive examples and counterexamples.