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. a reasonably clear picture has emerged of the patterns and causes of ocean currents B. a reasonably clear picture of the patterns of ocean currents and their causes have emerged C. a reasonably clear picture emerged of ocean currents, their patterns and the causes of them D. there have emerged a reasonably clear picture of the patterns of ocean currents and what caused them E. there had emerged a reasonably clear picture of the patterns of ocean currents and their causes 末尾的It is only ...... that .....后面有什么要求吗？要倒装吗？选了E好像没有说过，请教 | 
