GWD 17-32 陈年老题请教

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: A

请教NN：of the patterns and causes of ocean currents 为什么在这里emerge了？

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 (of the patterns and causes of ocean currents）has emerged (of the patterns and causes of ocean currents.)

把of the patterns and causes of ocean currents放在句末因为has emerged 谓语太短、主语太长，有头重脚轻之嫌。

至于为什么用完成时，文中的意思是直到...才有..，强调完成，跟Navigators have known 前一个东西对应

回楼上：in the last half century正是完成时的标志，"大段时间"

楼上很好的解释