大N大N帮帮我，快考试了，求解若干语法题

In 1981,children in the United States spent an average of slightly less than two and half hours a week doing household chores; by 1997 they had spend nearly six hours a week .

正确答案是chores; by 1997 that figure had grow to nearly six hours a week 但是没有任何理由用过去完成时的啊
  ，1981 在1997 之前，才用了一般过去式

我选的是 chores, whereas nearly six hours a week were spent in 1997

谁能帮我一下啊

 whereas前后尽量对称，而且被动式不好。关于时态的问题我这几天也被搞糊了，有些时候没有明显时间提示词的情况下确实会出现完成时态。

 whereas前后尽量对称，而且被动式不好。关于时态的问题我这几天也被搞糊了，有些时候没有明显时间提示词的情况下确实会出现完成时态。

20: GWD-18-Q19

New genetic evidence—together with recent studies of elephants’ skeletons, tusks, and other anatomical features—provide compelling support for classifying
   Africa’s forest elephants and its savanna elephants as separate species.

A.      provide compelling support for classifying

B.       provide compelling support for the classification of

C.      provides compelling support to the classification of

D.      provides compelling support for classifying

E.       provides compelling support to classify

为什么不选D？表目的应该用to do ，难道是provide 的习惯用法

就是选D啊？首先主语是单数形式，排除AB。provide sth for doing sth.选D

6. GWD21-Q6:

Heirloom tomatoes, grown from seeds saved from the previous year, only look less appetizing than their round and red supermarket cousins, often green and striped, or have plenty of bumps and bruises, but are more flavorful.

A.      cousins, often green and striped, or have plenty of bumps and bruises, but are

B.       cousins, often green and striped, or with plenty of bumps and bruises, although

C.      cousins, often green and striped, or they have plenty of bumps and bruises, although they are

D.      cousins; they are often green and striped, or with plenty of bumps and bruises, although

E.       cousins; they are often green and striped, or have plenty of bumps and bruises, but they are

答案是E，为什么不是B啊

加了although（让步语气）进去句子重心有点改变。另外although后面好像要是个完整句子（SVO）。