GWD-17-32
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
ans: A
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A是怎麼平衡那個and的?
看不太懂... (?) and (causes of ocean currents)