While this is a Strengthen question, it has a very paradox-esque structure. It seems odd that despite the fact that the trash-to-energy plant produces three times more air pollution than the natural gas plant, the environmentalists unanimously support the implementation of a trash-to-energy plant over the natural gas plant.
When predicting a question like this, I am trying to think about some way that this trash-to-energy plant would be better for the environment in comparison with a natural gas plant. The only thought coming to my mind is the fact that is removing trash from the environment. Let's see if that plays a role.
Answer C provides a variant on this thought. If the new trash-to-energy plant replaces an incinerator that is far worse than the plant itself, than it is playing a very positive role for the environment, as opposed to the natural gas plant that would presumably not replace the trash incinerator.
Comparing new vs. old natural gas plants does not help us determine why, despite the greater air pollution of the trash-to-energy plant, the environmentalists still preferred it to the natural gas plant.
I hope this has helped, please let me know if you have any other questions.