请大家帮帮忙！！GWD17-Q32的A选项！

quellekind

Navigators have known for thousands of years that the ocean has variable currents, but it is only in the last half century that a reasonably clear picture has emerged of the patterns and causes of ocean currents.

 

A.      a reasonably clear picture has emerged of the patterns and causes of ocean currents

 

B.       a reasonably clear picture of the patterns of ocean currents and their causes have emerged

 

C.      a reasonably clear picture emerged of ocean currents, their patterns and the causes of them

 

D.      there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them

 

E.       there had emerged a reasonably clear picture of the patterns of ocean currents and their causes

这题答案说是A，但我不太明白为什么时态上要用has？直接用过去时不会更好么？期盼各位的解答~~

newgoldrs2

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hellower

In B,D,E, there are no clear and reasonable referent for "their", which can

refer back to "the patterns" or "ocean currents". Construction in C is wordy and

unidiomatic as well.

amorer

我光荣的选成了C也是想着应该是一般过去时。但现在看起来根本不通。赶时间真是急剧提高错误率。

个人认为一般过去时和完成时都是可行的。

quellekind

原来有这种知识点啊...谢谢NN指点~

initusur

完成时的使用有强调的意味
这里先强调了Navigators have known for thousands of years　明显后面要进行对比，即强

调时间上的差异性，所以用完成时
过去时只是平白叙述，没有强调意味。