Dear hellother,

The main purpose is to get the answer - roads can be many. You select the one which you're comfortable with.

For example, read following CR question -

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 seriously weakens the plan's chances for success?

(A) The cost of raising soybeans has increased significantly over the past several years and is expected to continue to climb.
(B) Tests of a newly developed, inexpensive pesticide have shown it to be both environmentally safe and effective against the insects that have infested cotton crops.
(C) In the past several years, there has been no sharp increase in the demand for cotton and for goods made out of cotton.
(D) Few consumers would be willing to pay significantly higher prices for cotton goods than they are now paying.
(E) The species of insect that has infested cotton plants has never been known to attack soybean plants.

Here, we have to find an option which weakens the argument.

Taking the options one by one, following procedure helps eliminate the options one by one. In the end you are left with a choice from either B or C. Then you have to read both of them carefully and decide which one weakens the argument more.

As per the passage, what is the plan ? - To grow cotton instead of Soyabean to reap profits.

A: Strengthens (as this encourages the farmer to grow cotton & go as per plan)
B: Weakens (As a cheap pesticide will kill the insects that infested Cotton, it will bring the prices of Cotton down very soon. So the plan to grow cotton may not prove to be a successful one.)
C: Weakens (As this may bring down the price of cotton and discourage the farmer to grow cotton as per his plan.)
D: Strengthens (as this encourages the farmer to grow cotton & go as per plan)
E: Strengthens (as this would keep the prices of Soyabean low. Thus farmer would not be encouraged to grow Soyabean)

From above arguments, comparatively, B weakens the argument more than C.
Thus ans is B
Hope this has helped you and answered your question.