大全968的时态问题，望nn们指点

wjjjojo

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 but have remained virtually unchanging’

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

答案e 我选a 

 关于时态的问题，
        为什么两个并列结构中的时态不同，为什么这么用？？

wjjjojo

又没人理，我自己顶

期待蔚蓝

以下引用别的帖子的发言：

6 months是在3 months之前的一个时间段, 3 months是延续到现在的时间段, 6 months在它之前只能用过去时。

现在完成时态通常表示动作延续至今，但是从句义上很明显看出，前面的动作是发生在至少3个月前的，所以，前半句应该用一般过去时

现在完成时表示的是某一个动作持续到现在并且还有可能继续持续下去。during the first six months of this year 很明显是表示过去的一段时间，declined 这个动作也已经完成了，所以不可能用完成事态的，所以应该用过去时during the next three months可以用完成时态，来表示动作持续下去的趋势...