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) Rivers, originally flowing into the St. Lawrence by way of Lake Michigan, have been redirected through the construction of
OA：E
我的问题是：选项B的解释说不能用过去完成时。但是另一个PREP的正确句子是：
Ozone, a special form of oxygen that screens out harmful UV rays, reaches high concentrations twelve miles above Earth, where it had long appeared immune to human influence; we have now realized, though, that emissions of industrial chloroflorocarbons deplete the ozone layer.
为什么上面这里需要用过去完成时来突出对比，而上题中B选项用过去完成时确实错的？
求各路神仙指教啊！ | 
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发表于 2011-12-5 22:38:02
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