亲们，看过来GWD17-Q32

GWD17-Q32:

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

为啥选A呢？？ 谢谢谢谢大家，告诉告诉俺吧(搜过历史贴）