"Oil spills affect water in a variety of ways. When oil is released into water, it does not blend with the water. Oil floats on the surface of salt and fresh water. Over a very short period of time, the oil spreads out into a very thin layer across the surface of the water. This layer, called a slick, expands until the oil layer is extremely thin. It then thins even more. This layer is called a sheen and is usually less than 0.01 mm thick."
I'm choosing this option because it makes the most logical sense. The spread of oil in an oil spill is affected by the temperature of the water, so the oil will in turn change the temperature of the water.
Or at least that's what we learnt in physics last week
