I think that the best answer is
B people were not careful about preventing waste and contamination, although one could also find good arguments in favour of A and C.
A big problem was that oil was very easy to find, so people didn't protect every source and were often wasteful, letting the oil spill to the ground.
Taking precautions to prevent pollution is always more costly than not doing it, so it could be argue that this did not change, and is therefore not applicable as an answer to this question.
