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.