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
这题答案说是A，但我不太明白为什么时态上要用has？直接用过去时不会更好么？期盼各位的解答~~ | 
