We accept your order and are enclosing you our sales contract in duplicate of which please countersign and return one co
问题描述：
We accept your order and are enclosing you our sales contract in duplicate of which please countersign and return one copy to us for our file.就是which前为什么有of呢
答
我们接受您的订单,我们附上你的销售合同一式两份敬请连署并寄回我方存档一份给我们谢谢您的回答，可是我知道这个句子的意思，只是不明白which前得那个of是哪里来得of可有可无的