Hello goodyear, I am happy to help.
First lets break down the stem. Let ourselves be the farmers of developing countries. What are we saying?
1) We say that American govt offers subsidies for their farmers through buying all the additional crop that is left unsold. So American farmers aren't controlling their output
2) Because there is more production for a particular demand, the prices of wheat are coming down.
3) Conclusion: Without subsidies, American farmers will occur loss for any crop unsold and so they will start producing what is needed, hence increasing the wheat price (for a given demand if the production is less, the prices will rise)
So what can be said in order to shut our mouths criticizing American govt?
A) In the entire topic we are not talking about increasing or decreasing demand. We were concerned that whatever the demand is American farmers are producing more, hence bringing down the price of wheat. So reject this option.
B) Non-government buyers are able to predict the demand for wheat. Again, amount of demand is not the topic under discussion. Reject
C) Soybean? Do you really think showing an example of soybean is going to stop us from making the same statement? How can you compare Soybean and wheat? Their demand, production models may be different. Reject
D) Even if America stops its production, other countries will produce more wheat. Whether it is America or other country, we do not bother. Our concern is wheat is produced more than demand and so prices are lower. In any case, some country or the other is producing more wheat, so how can our attack on American to stop subsidizing (more wheat production) is going to help our case? Contender
E) Follows the same reasoning as C. Reject.
So D) is the answer.
You can also look in to this question as a cause and effect model.
Cause= American subsidies are encouraging more production of wheat
Effect= Prices are lesser
If we can find another cause to the effect we can weaken the argument.
Another cause is not just America, even Canada and Russia produces more wheat. So again D is the answer.