It is crucially important to farmers that the herbicides they use to

Conclusion: Herbicides are safe for corns while they are lethal for soyabens. Since, Corns and Soyanbeans are not sown in the same field in the same year, Herbicides could be used on Corn.
To weaken this argument we need a statement that could prove that herbicides once used on corn in a field also have some effect on Soyabeans although they are sown in different years.
Answer choice C best accomplishes the work. If herbicides persist in a field for over a year then they would have some effect on Soyabean sown in the same field in the next year. Hence, C is the answer.

Premise :- soybeans and corn are frequently sown in a field in alternate years.
Conclusion :- in such fields, the herbicide ( Which is lethal to soybeans ) can safely be used on the corn, since the two crops are not sown together in the same field.

To weaken this conclusion we need to show that " in such fields, the herbicide ( Which is lethal to soybeans ) can NOT safely be used on the corn, since the two crops are not sown together in the same field."

Option C does that job.

If some of the herbicide persists as a residue in the soil for over a year then it may kill soybean in the following year ( after the year when corn was sown). So in such fields, the herbicide ( Which is lethal to soybeans ) can NOT safely be used on the corn.

Please give me kudo s if you liked my explanation.

Soya / Corn + Soil + Herbicide = Effective weed control

Soya<--------not good-- Herbicide -good ---------> Corn

Now , except Soil all other factors may change , so answer has to be (C)

The author of the passage tells us that farmers want to protect their crops from weeds but it also possible that the herbicides used for killing those weeds unintentionally harm the crops itself. But such a situation is normally avoidable in case of a herbicide because it can be used to kill weeds in corn and soybean crops and at the same time not harm soybean crops,which are very sensitive towards it, as the corn and soybean are grown in alternate years and not together.

But, I say what if somehow, whatever herbicide you put in corn this year, actually happens to stay in sufficient quantity in the soil in the next year also. In such a case, soybean crop will die, won't it ?

With such an idea in mind, we can examine the options :

1. Okay so this herbicide is good with our insects and birds. But does that mean it will harm soybean crop in anyway ? We are not getting any idea about such a scenario. Hence , this option doesnt impact author's conclusion. Incorrect choice.

2. Well..for sure this would have been the case, otherwise why would there be a need to worry about the soybean and herbicide clash. This option doesnt seem to lay doubts on author's conclusion in anyway actually. Has no impact whatsoever. Hence, incorrect choice.

3. This option hints that the herbicide might stay in the soil when time to grow soybean has come. Obviously, there will be a clash then and soybean might die. In such a case , the author's conclusion seems to shake.
So correct choice.

4. This option tells how to use herbicide nicely for corn crops. But does it tell us how dosing herbicide sooner will affect soybean crop which has to be grown next year ?
It surely doesn't and hence impacts the author's conclusion in no way.
Incorrect choice.

5. Okay.. so that's good for the case in hand. In short, soybean crops might not be exposed to higher quantities of herbicide in general. But does this option tell us how growing the two crops in alternate years and not together might still actually lead to soybean crops dying eventually ? .It doesn't and so has no impact on author's conclusion. The conclusion still stands as it is. Hence incorrect choice.

Answer : C

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herbicide used: safe for corn, lethal to soybean
conclusion: use this herbicide can be safe for corn

weaken prethink: using this herbicide cannot be safe for corn for some reasons

(a) The herbicide has not been shown to harm insects, birds, or other animals that come in contact with it.
harm insects, birds, or other animals is irrelevant to answer "is this herbicide safe for corn?"
(b) Scientists have so far failed in their attempts to breed soybeans that are resistant to the herbicide.
breed resistant-to-the herbicide soybean is irrelevant to answer "is this herbicide safe for corn?"
(c) Some of the herbicide persists as a residue in the soil for over a year.
residue can do harm to crop, so corn is at risk, cannot be safe anymore, bingo!
(d) To be effective, the herbicide must be applied early in the growing season of the corn.
time to apply can answer "is this effective?" but still cannot answer "is this herbicide safe for corn?"
(e) Soybean crops do not require the application of herbicides to the same extent as do corn crops.
but they are frequently sown in a field in alternate years. if the soybean do not require herbicides, it may be safe for the soybean but whether be safe for corn is unknown.