Country W has banned importation of oranges from Country V

To know what cast the doubt, we need to find what will make the ban unsuccessful.
Premises: To protect orange growers in country W, importation of oranges from country V was banned.
Conclusion: The ban is considered a success since orange prices in country W have risen.
Explanation: orange growers in country W could not sell at a good price due to the importation from country V.

Option A - Incorrect
This does not affect the ban. The conclusion is not about country V but country W.

Option B - Incorrect
In as much as it does not change the rise in price of the oranges, it also does not cast any doubt to the conclusion.

Option C - Incorrect
The conclusion is not concerned about country V.

Option D - Correct
This causes a doubt because the conclusion given in the argument is the cause to the decline in sales of the oranges.

Option E - Incorrect
This is outside the scope of the argument

What we see: a weaken question. The argument says the ban on Country V's oranges is a success because orange prices in Country W went up.

Step 1. Find the goal, the evidence and the conclusion.
What we see: goal of the ban, "protect Country W's orange growers." Evidence, prices rose. Conclusion, the ban is a success.
What it means: the argument treats higher prices as proof that the growers are better off.
What we do and why: that is the gap. Higher prices help growers only if they still sell enough oranges. A fact that breaks that link weakens the argument.

Step 2. Look for the choice that breaks the link.
The choice that says higher prices will lead to fewer sales of those oranges does it. If growers sell fewer oranges, the higher price may not leave them any better off, so rising prices are no proof the growers were protected.

Step 3. Check the rest.
A: demand in Country V is about the other country. It says nothing about W's growers.
B: lower demand in W would push prices down, yet prices went up. It does not touch whether the growers gained.
C: a freeze in Country V. V's oranges are banned from W anyway, so V's crop cannot affect W's prices or growers.
E: growers save advertising money. That helps the growers, which makes the ban look better.

The trap: C. It looks like another cause for the price rise, but oranges from V cannot enter W under the ban, so V's harvest does not matter here.
Takeaway: When a plan is judged a success by one number, ask whether that number really shows the goal was met.

Answer: D