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.

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

末尾的It is only ...... that .....后面有什么要求吗？要倒装吗？选了E好像没有说过，请教

ANS:A??

What is the correct ans ???

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

B 的their指代不清。不知道是指代patterns还是currents。

C their patterns and the causes of them 不合语法

DE there不能紧跟在that后。

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

B 的their指代不清。不知道是指代patterns还是currents。

C their patterns and the causes of them 不合语法

DE there不能紧跟在that后。