B – Wrong
This option is evidence that the worst has not happened yet and the damage is still being done. So, B is not the answer.
C – Wrong
This is not suggested by the passage and so, is irrelevant
D – Wrong
I understand why a lot of the people may think D could be the answer as the passage clearly states that the harmful effects are greater now than before. However, it does not summarise the entire passage as well as A does. The passage tries to say that the worst is yet to come. D only says that the effects are greater now.
E – Wrong
This option only says the amount of time the oil will remain on the beaches. However, the passage is mainly focused on saying that the worst effects are yet to come.