She struck the keys too hard and two of the strings were broken.这句话里面two 后面为什么要加of呢?

She struck the keys too hard and two of the strings were broken.这句话里面two 后面为什么要加of呢?
She struck the keys too hard and two of the strings were broken.这句话里面two 后面为什么要加of呢?

She struck the keys too hard and two of the strings were broken.这句话里面two 后面为什么要加of呢?
按键按得太用力了,弄坏了两根弦,不加of也没大问题,但弦肯定不只两根,two of就是强调弄坏的“其中”两根弦,这样比较顺口和地道吧