2013年4月20日，四川省雅安市芦山县发生了7.0级地震，某校开展了“雅安，我们在一起”的赈灾捐款活动，其中九年级二班50名学生的捐款情况如下表所示：

     The children often play soccer in the garden(花园). Sometimes they break (打破).Mrs. Smith’s windows.  
One afternoon , Mrs. Smith is reading a newspaper at home. After a short time, she closes her eyes and goes to sleep. At that time a little boy knocks(敲) at the door. Mrs. Smith opens the door and asks, “Do you break my windows again（又一次）?” “Oh, no”, says the boy. “ Your windows are open this time and our soccer is in your room. Can I get it back, please?” 
根据短文内容，选择正确答案。

有三堆棋子，每堆棋子42枚，并且只有黑白两色棋子．第一堆里的黑子和第二堆里的白子一样多，第三堆里的黑子占$\frac{3}{7}$，把这三堆棋子集中在一起，白棋子占全部棋子的（　　）