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

of the patterns and causes of ocean currents为什么放在emerge后面呀？