Which of the following is a direct cause of surface currents?
a.
uneven heating
b.
high salinity
c.
differences in density
d.
wind forces

Surface currents are largely and generally created by Wind and these are determined by wind direction. Due to the rotation of the Earth, Coriolis forces and the interaction of currents with landforms create surface currents. So I would go with the Answer D as wind forces are the major cause surface currents. However, there are times when density difference in water masses caused by temperature and variation in salinity trigger surface currents.

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