when's someone knocked at the door, Jack was making a telephone call.这里面的when's
问题描述：
when's someone knocked at the door, Jack was making a telephone call.这里面的when's
怎么解释啊,这个句子是不是错了啊
答
当有人敲门的时候,JACK正在打电话.when后面's应该没有吧