请教大全天山新题另外一道

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

我只想知道during the first six months of this y与during the next three months与其对应的时态用法。。。

谢谢！

能不能把全题粘贴。。。

during一般用完成时态。。

注意 but是连词，如果你这个句子是对的。。。那注意 remained前面省略了 have