GWD第十四套GWD17-Q32:求助

笨蛋加油

32. 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：A
哪位大侠能帮我解答一下A是个什么结构？我看出来貌似是个强调句，has emerged of  和 causes是并列的吗？我怎么看不懂句意啊？

笨蛋加油

明白了。。。
句子还原为：（a reasonably clear picture of the patterns and causes of ocean currents） has emerged
不好意思啦O(∩_∩)O~

17tomato

这…… 这上考场了怎么分辨啊，Lz要是不说我估计死也选不到A……