How can oceanographers measure surface currents from space?

How can oceanographers measure surface currents from space?
 
  a) Satellite pictures can show objects on the sea surface drifting around.
  b) Satellites can measure wind speed and direction, which can then be used to predict current direction.
  c) Satellites can detect induced electromagnetic currents caused by a moving conductor (seawater) in Earth's magnetic field.
  d) Ekman transport and current motion cause the water surface to bulge, and the departure from normal sea level can be measured by satellite radar.
  e) They cannot; you have to be near the water to see which way it moves.