GWD 17-32 我看不懂这题的意思，能帮我翻译一下吗？

Aaroncy

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 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

there had emerged a reasonably clear picture of the patterns of ocean currents and their causes

Answer: A

从emerged那里开始看不懂了。。。。