I'd say the answer is C because that doesn't really narrow down what he is trying to write about. he needs a more complex question for research.
D.Time-consuming to develop is the chosen one.
It depends on how you worded your essay and what your views on global warming are, the way I would word it is "in conclusion global warming is real and changing drastically every minute, and we don't fully know exactly how it will affect the world long-term" that's one way you can say it.
