(2015.•广东佛山)I don't understand why more girls ________ to do housework than boys in today's society.
A:asked
B:were asked
C:are asked
① (a+b)2=a2+2ab+b2
② (a−b)2=a2−2ab+b2
③ a2−b2=(a+b)(a−b)
④ (a+2b)(a−b)=a2+ab−2b2
氯化钠在20℃时的溶解度为36克.则20℃时,氯化钠饱和溶液中溶质、溶剂、溶液之间的质量比正确的是( )