An oil spill is an example of

A release of oil into the environment, either leaked or discharged from a ship, lorry, truck etc
The major environmental impacts so far have come from normal activities, rather than catastrophic oil spills.
Or
an accidental release of oil into a body of water, as from a tanker, offshore drilling rig, or underwater pipeline, often presenting a hazard to marine life and the environment.

The correct answer would be b) volume versus temperature, because they are also directly proportional.