Deep tillage is even more deleterious to the world's topsoil supply

­Deep tillage is even more deleterious to the world's topsoil supply than previously believed. For example, farmers who till deeply are ten times more likely to lose topsoil to erosion than are farmers who use no-till methods. Results like these make it clear that farmers who now till deeply should strive, by using other topsoil aeration techniques, to incorporate no-till methods instead.

The main conclusion of the argument is the following:

farmers who now till deeply should strive, by using other topsoil aeration techniques, to incorporate no-till methods instead

The main conclusion is supported by the following intermediate conclusion:

­Deep tillage is even more deleterious to the world's topsoil supply than previously believed.

which is suppported by the following example:

For example, farmers who till deeply are ten times more likely to lose topsoil to erosion than are farmers who use no-till methods.

We see that the author of the argument has observed a correlation between deep tillage and increased topsoli erosion, "farmers who till deeply are ten times more likely to lose topsoil," and concluded, essentially, that deep tillage causes increased topsoil erosion and therefore should be replaced with no-till methods.

The argument depends on assuming which one of the following?

This is an Assumption question, and the correct answer will state an assumption necessary for the premise to support the conclusion.

(A) Topsoil erosion does not make farmers want to till more deeply.

This choice is a little hard to eliminate because it could seem to present an alternative cause-effect relationship. We might think that the relationship may not be that deep tillage causes erosion but that erosion causes farmers to want to use deep tillage.

Notice, however, that this choice does not say that erosion does not cause farmers to use deep tillage rather than another method. It says that it doesn't cause them to want to till "more deeply." So, maybe they are already using deep tillage when the erosion begins.

Honestly, in a GMAT question, this choice might be correct. However, in an LSAT question, little differences such as "deep tillage" versus "till more deeply can make a choice incorrect. 

Eliminate.

(B) In deep-tillage farming, the deeper one tills, the greater the susceptibility to topsoil erosion.

The argument does not involve any claims about the effects of tillling deeper.

Also, regardless of whether this choice is true, it remains the case that more soil erosion occurred when deep tilling was done, and that evidence supports the conclusion.

So, this argument doesn't depend on this choice.

Eliminate.

(C) Tilling by any method other than deep tillage is not a viable option.

This choice is interesting.

The argument goes from the fact that deep tillage is associated with greater topsoil erosion to the conclusion that "farmers who now till deeply should strive, by using other topsoil aeration techniques, to incorporate no-till methods instead."

Notice that, in working that way, the argument relies on the assumption that there is no other option. After all, if there is another option, then it doesn't make sense to conclude that farmers should jump to using no-till methods instead. Rather, they could switch to using another method of tilling.

So, the argument relies on it being true that there's not another viable tilling method.

Keep.

(D) The most expensive farming methods employ topsoil aeration techniques other than deep tillage.

If anything, this choice is a reason to use deep tillage to minimize expense.

Since the author is arguing for ceasing to use deep tillage, if anything, this choice goes against the argument.

Eliminate.

(E) On average, topsoil that is no-tilled is more aerated than topsoil that is tilled deeply.­

The conclusion of the argument is not about using no-till methods to achieve more aeration. It's about using no-till methods to prevent erosion.

So, regardless of whether this choice is true, the argument works.

Eliminate.

Correct answer: C­