请问The ladder,however,was so short that it is coudn't reach t

The ladder,however,was so short that it is couldn't reach the girl。
译：可是,这梯子太短了,够不着那个女孩。
再问： 为什么是it is couldn't呢
再答： 答：这个is是多余的，应该是出题的疏忽。