prep2-32

32.The Chicago and Calumet Rivers originally flowed into the St. Lawrence by way of Lake Michigan, but having been redirected by constructing canals so that the water now empties into the Mississippi by way of the Illinois River.

(A) Rivers originally flowed into the St. Lawrence by way of Lake Michigan, but having been redirected by constructing

(B) Rivers had originally flowed into the St. Lawrence by way of Lake Michigan, but they have been redirected by constructing

(C) Rivers, which originally flowed into the St. Lawrence by way of Lake Michigan but have been redirected by the construction of

(D) Rivers, originally flowing into the St. Lawrence by way of Lake Michigan, but having been redirected by the construction of   (E)

(E) Rivers, originally flowing into the St. Lawrence by way of Lake Michigan, have been redirected through the construction of    

In E, flowing into the St. Lawrence .... is  present tense, however, the rivers have changed their courses. here, i think that it should use a past finished tense. so i choose B.

what wrong with B?

but they have been redirected by constructing，河流是不可能construct什么的

E没错，现在分词修饰完全正确

Further to upstairs, you should note the past perfect tense.

我不是很明白在此处,为和用+ING 形式.   , originally flowing into the St. Lawrence by way of Lake Michigan,

8.         (24773-!-item-!-188;#058&001691)

Ozone, a special form of oxygen that screens out harmful ultraviolet rays, reaches high concentrations twelve miles above Earth, where it has long appeared that it was immune from human influence; we have now realized, though, that emissions of industrial chlorofluorocarbons deplete the ozone layer.

(A) has long appeared that it was immune from

(B) has long appeared to have been immune from

(C) has long appeared as being immune to

(D) had long appeared immune to

(E) had long appeared that it was immune to

这一题之所以选D，是因为后面的分句用了have now realized，与后面的句子的时态相比，前半句说ozone已经不是这个时候（have now realized的时候）之前的那个时候人们所认识的了，所以用了had long appeared。

那么楼主这道题，为什么B中的时态是错误的？

PREP 的题目是有点大怪路子 啊

虽然同意答案是E,也同意楼上对by v-ing的说法

但是E里面这个分词用的也满奇怪的,原句中用originally flowed , E里面却将谓语动词变成了分词作状语, 似乎有点改变意思的味道