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.

• Aa reasonably clear picture has emerged of the patterns and causes of ocean currents  
• Ba reasonably clear picture of the patterns of ocean currents and their causes have emerged  
• Ca reasonably clear picture emerged of ocean currents, their patterns and the causes of them  
• Dthere have emerged a reasonably clear picture of the patterns of ocean currents and what caused them  
• Ethere had emerged a reasonably clear picture of the patterns of ocean currents and their causes