【正确答案】 C

【答案解析】ColumnB: a(b + a)-b(a + b)=
ab + a²-ba-b² = a²-b².
Column A: (a + b)(a -b) = a² -b².

【正确答案】 D

【答案解析】If x and y represent the number of 5-cent stamps and 7-cent stamps, respectively, that Dalia used, then 5x + 7y = 75. There are infinitely many solutions to this equation, but there are only two solutions in which x and y are both positive integers: y = 10 and x = 1 or y = 5 and x = 8. Neither column is always greater, and the two columns are not always equal (D).