Erica,
Rather than trying to eliminate wrong answers, it is far faster and more efficient to prephrase the right answer (though this is a skill that is difficult to master). The argument seems to say that since native grasses absorb CO2 better than trees, incentives to plant more trees hasten global warming. This argument seems extreme. Why? Well, just because a square meter of trees might eliminate 10 tons of CO2 per year and a square meter of native grasses might eliminate 12 tons per year doesn't mean that planting trees makes global warming worse. Unless...
Unless of course more trees meant less grass. So that is my prephrase. Answer D is the only one that talks about both trees and native grasses and does seem to say that planting trees displaces grasses, which are better.
Now to your question about B. It is not relevant. We already accept the idea that the government incentives lead to more tree planting. That is not in dispute. What is disputed is whether that is overall good for removing CO2. Of course each tree removes CO2 but if it replaces native grasses remove more, it could be the less good choice.
We don't always need to analyze every answer choice with unless using conditional reasoning (only most of the time!
) but when you also need to negate it for Assumption Negation purposes, then you DO need to carefully diagram and negate. Hope that helps.