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.

1. a reasonably clear picture has emerged of the patterns and causes of ocean currents
2. a reasonably clear picture of the patterns of ocean currents and their causes have emerged
3. a reasonably clear picture emerged of ocean currents, their patterns and the causes of them
4. there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them
5. there had emerged a reasonably clear picture of the patterns of ocean currents and their causes

   Answer: is A    why? thanks

B。主谓不一致。picture...has emerged

C。their,them指代不明

D。E就不说了吧

我觉得A的时态不一致

B的主语是pictures and causes 用have emerged是没有错的呀

NN们请教!

B的主谓没错，但their可以指代Patterns也可以只待causes，所以错.

b主谓不一直呀，主语picture...谓语have emerged
C的表达有问题吧picture emerged of ocean currents，应该是picture of

ocean currents emerged吧

B的主语应该是pitcure,因为有of存在，所以应该连接their causes。

如果硬要理解成pictures and causes 就会产生逻辑问题。文中是说近半个世纪pictures出现，而causes是个客观现象，是一直存在的，不是近半个世纪才出现的。

还有这里的their我觉得指代没有问题，应该指代currents。首先符合逻辑意思，其次根据语法规则代词优先指代主语中心词。

E,大家能讲讲为什么不对么？it is.....that...强调结构；there had

done+主语  倒装结构；而且E还回避了A中的 the patterns and causes of

ocean currents的歧义问题，为何不选择E

另外，同意B选项从句主语是picture所以have 不对！