31. 對於任意正整數 a、b、c,問下列哪一個等式恆真? (A) (a ÷ b) × c = a ÷ (b × c) (B) (a ÷ b) ÷ c = a ÷ (b × c) (C) (a − b) − c = a − (b − c) (D) a ÷ (b + c) = (a ÷ b) + (a ÷ c)