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: is A    why? thanks

B。主谓不一致。picture...has emerged

C。their,them指代不明

D。E就不说了吧