（1）     GWD-17-Q32

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

答案是A怎么解释呢？划线部分翻译过来是什么意思呢？