题目

完形填空。从A、B、C、D四个选项中选择一个最佳答案    One day, some workers were mending the road. They dig a big hole in the road. As they could not  1  their work, they put up notice boards on both sides of the hole before they  2 . On the boards they wrote the word DANGER in order to warn people not to drop into the hole.    Some time 3 , a blind man was walking  4  the road. He was just walking in the direction of the hole. As he could not see the notice, he was walking  5  to the hole. When he was very close to it, a boy saw the man and  6  up to him. 生物分类的等级从高到低依次是：界、（______）、纲、目、（______）、属、（______）。