大全968，怎么看怎么不平衡，时态问题

968。  Interest rates on mortgages have declined steadily during the first six months of this year but virtually remained unchanged during the next three months.

(A) have declined steadily during the first six months of this year but virtually remained unchanged

(B) declined steadily during the first six months of this year but virtually remain unchanged

(C) steadily declined during the first six months of this year but remain virtually unchanged

(D) declined steadily during the first six months of this year but have remained virtually unchanging（E）

(E) declined steadily during the first six months of this year but have remained virtually unchanged

查了大全注释，觉得解释有点自相矛盾。E选项两个并列都是用during表时间期限，前一个用过去时，后一个用完成时，不是明显的不平衡吗？

求教NN。。。。