Surface currents are on the
of the Earth's oceans

Surface currents, which make up only 8% of all water in the ocean, are generally restricted to the upper 400 m (1,300 ft) of ocean water, and are separated from lower regions by varying temperatures and salinity which affect the density of the water, which in turn, defines each oceanic region.

Ithink your answer is d

Some type of thing that was invisible