In several countries, to slow global warming, many farmers are planting trees on their land because of government inc...

C. Ann on March 25, 2015

Necessary Assumption

Could you explain the negation technique to locate the correct answer for necessary-assumption questions? Thanks,

1 Reply

Melody on March 28, 2015

So, I would recommend revisiting the Strengthen with Necessary Premise lesson if you feel you are a little weak on the techniques. We also have a few blog posts on "the logical opposite" on our blog that you may find helpful in negating these sentences (visit lsatmax.com/blog and search "logical opposite"). Once you've done that, come back to this problem and walk through this explanation.

Alright, remember that a premise is necessary for a conclusion if the falsity of the premise guarantees or brings about the falsity of the conclusion. First we check to see if the answer choice strengthens the passage, and then, if it does strengthen, we negate the answer choice to see if its negation makes the argument fall apart. If the answer choice does both those things then it is our correct answer.

So, what is our conclusion? "Therefore, these incentives are helping to hasten global warming."

Why? Well, in an effort to slow global warming, several countries offered government incentives to many farmers to plant trees on their land. These incentives arose from research showing that vegetation absorbs carbon dioxide, which might otherwise have trapped heat in the atmosphere. However, a recent study shows that trees absorb and store carbon dioxide less effectively than native grasses.

Your first thought should be, just because native grasses are more effective in absorbing and storing CO2, why does that mean that planting trees that still absorb and store CO2 will HASTEN global warming?

Aren't the trees still helping slow global warming by absorbing and storing the CO2? Yes, unless the trees that are being planted are in some way detrimental to native grasses that would otherwise be more efficiently solving the issue of slowing down global warming.

This brings us to answer choice (D): "some of the trees planted in response to the incentives are planted where native grasses would otherwise be growing."

Bingo.

Does this answer choice strengthen the argument? Yes.

If some of these trees that are being planted in response to the incentives are being planted in places where native grasses would otherwise be growing, the trees are actually harming a type of vegetation that work more efficiently at absorbing and storing CO2, which would otherwise be trapped heat in the atmosphere.

So, answer choice (D) most definitely strengthens the conclusion of the argument because the government incentive for planting trees is helping to hasten global warming by not allowing native grasses, which do a better job at mitigating global warming, to grow in as many areas at it otherwise would be.

Now, does the negation of this answer choice break the argument down? Yes.

Negation: none of the trees planted in response to the incentives are planted where native grasses would otherwise be growing.

NOTE: "some" means "at least one." So, it's logical opposite, i.e. negation, would be "none."

In this case, if none of the trees would be planted where native grasses would otherwise be growing, then the government incentive of planting trees would not hinder the goal, i.e. hasten global warming. The native grasses would grow where they already were growing and the newly planted trees would only add to helping the native grasses in absorbing and storing CO2.

Therefore, if we negate answer choice (D), the conclusion that these incentives are helping to hasten global warming no longer stands.

Hope that clears things up! Please let us know if you have any other questions.