32. GWD17-Q32:
Navigatorshave known for thousands of years that the ocean has variable currents, but itis only in the last half century that a reasonably clear picture has emergedof the patterns and causes of ocean currents.
- a reasonably clear picture has emerged of the patterns and causes of ocean currents
a reasonably clear picture of the patterns of ocean currents and their causes have emerged
a reasonably clear picture emerged of ocean currents, their patterns and the causes of them
there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them
E. there had emerged areasonably clear picture of the patterns of ocean currents and their causes
能解释一下这题为什么A正确么？in the last half century不是一个确定的时间点嘛，为什么可以用完成时？ | 
