Insect infestations in certain cotton-growing regions of the world have caused dramatic increases in the price of cotton on the world market. By contrast, the price of soybeans has long remained stable. Knowing that cotton plants mature quickly, many soybean growers in Ortovia plan to cease growing soybeans and begin raising cotton instead, thereby taking advantage of the high price of cotton to increase their income significantly, at least over the next several years.

Which of the following, if true, most calls into question the reasoning on which the plan is based?

Now, you have mentioned that you had a tough time deciding between option B and C. Before we go on to discussing these options, let’s simplify the information given in the prompt :

1. Price of cotton has increased dramatically in certain regions of the world
2. The reason behind the price is insect infestation in those regions
3. In contrast to the above situation, the price of soyabean has remain steady for long now
4. Because cotton plants mature quickly, many soyabean growers (in a place called Ortovia) plan to grow cotton instead of soyabean
5. Reason for the above: they want to cash in on the situation mentioned in points 1 & 2 above.

As per the question-stem, we need to find a new piece of information that will weaken the plan of soyabean growers in Ortovia. In order to arrive at the correct choice, let’s take a closer look at the facts:

Plan: Grow cotton instead of soyabean atleast for the next several years.

Basis for the plan: Cash in on the significant increase in the price of cotton.

Reason for the price-rise: Insect infestation.

Now, how do you think the insect infestation led to the price rise? Simple: by creating a gap between the demand for and supply of cotton. This means that the regions that have seen a dramatic increase in the price of cotton have considerably more demand for cotton than is being currently met. Now the soyabean growers have not assumed any increase in demand; they have only taken for granted that the shortage in the supply of cotton will persist for the amount of time they want to grow cotton. This is why they think they'll be able to supply cotton at the current high prices over the next several years.
And that's where answer choice B hurts the cotton farming aspirants in Ortovia. Answer choice B says:

(B)Tests of a newly developed, inexpensive pesticide have shown it to be both environmentally safe and effective against the insects that have infected the cotton crops.

If we factor in the new information given in this choice, we can see how over the next few years the problem of insect infestation may be taken care of and hence, there are chances that the prices will come down and the growers in Ortovia will not be able to take advantage of the price rise for long.

However, answer choice C says:

(C) In the past several years, there has been no sharp increase in the demand for cotton, and for goods made out of cotton.

This choice does not talk about the future. It talks about the situation so far. Now, even if the demand for cotton hasn’t increased in the past few years, it still is at a level that is not being met due to the insect infestation. So, even at the current level (without any increase in demand) there is a shortage of supply which the growers in Ortovia could very well fulfill if the shortage is not taken care of. Hence, this answer choice doesn’t do anything to hurt the plan of soyabean growers in Ortovia.

In RC questions, it's a good practice to spend some time thinking about the logic of the argument, with respect to the task given in the question, BEFORE diving in to the answer choices. This way one is able to watch out for any trap set in the wrong answer choices.

Answer: B