the two children bought some books( )the money they had saved .A with , B by

用with. “用”钱买书。

“用”什么工具，做什么。　to do sth. with sth.
这里后面的sth.应该是工具（能摸到的），如上面的题和下面这句：
He is writing a letter with my pen.
但用交通工具(by foot, by car, by train....)例外：
Because his home is far far away from the school, he go to school by air, everyday.

"用”什么方法，方式，做什么。to do sth. by doing sth.
He is writing a letter by copying one from the book "情话大全".