This answer choice has no impact on the argument. The main point is a conditional. If every country acted according to the theory, international trade would close.

Let’s say I tell you: If I go to Mars, I’ll be happy.

1. Does my statement tell you that I am going to Mars?
   No, right? That’s merely an if condition.
2. Let’s say someone tells you that Anish is not going to Mars. Would that change your belief in my statement “If I go to Mars, I’ll be happy”?
   My original statement is about what would happen if I go to Mars. Would I be happy or not?
   It is not about whether I’ll go to Mars. So, even if you learn that I am not going to Mars, my original statement will not be impacted by that new information.

The argument is about what would happen if a certain situation occurs. Whether the certain situation actually occurs or not is irrelevant.

The author does not assume anything about how the countries would actually be dealing with the theory. The commentator’s conclusion is about what would happen IF all countries acted according to the theory. How they actually act is irrelevant, and not necessary for the argument.

Let’s start with a variation of this answer choice.

(B’) No country blocks any of its markets to foreign trade.

What impact does this variation have on the argument?

This one is in line with the first gap I mentioned above. If no country blocks any market to foreign trade, there would not be any need to retaliate by closing some markets, and probably countries will continue to trade with each other. So, this variation weakens the argument.

You might be wondering: But if countries don’t close their markets to others, wouldn’t that mean that the countries are not acting according to the trade retaliation theory?

The answer is no.

Countries acting according to the theory would retaliate if there is a need to retaliate.

If there isn’t even a need to retaliate, they probably won’t. The theory is: if some other country closes some markets, you also close some markets to that country. If no country closes its markets to you, then perhaps you don’t close any of your markets to them.

Now, back to the original answer choice.

If this option is true, there is a chance that the initial trigger will not happen – if countries should not block, maybe they don’t block. So, the statement weakens the argument. Albeit, since the statement talks about what countries should do and not what they actually do, the impact is very marginal.

Since the statement anyway impacts the argument negatively, it can’t be an assumption.

Who should settle trade disputes is irrelevant to the argument and has no impact.

The argument never gets into settling trade disputes. The author does not express any preference either way. The argument does not rely on who should settle trade disputes.

This one fits.

This answer choice strengthens the argument. It tackles the first gap related to ‘triggers’ I mentioned above.

What would happen if for some pairs of countries, neither has any market closed to the other (negation)? In that case, even if these countries acted according to the theory, they will not have that initial trigger to retaliate to the other country by closing some of its markets to the other. And thus, these pairs of countries will probably continue to trade with each other.

Even if every country acted according to the theory, for trade to actually stop, there has to be a need to retaliate. If there isn’t even a need to retaliate (i.e. if a pair of countries has all markets open to each other), even if in principle the countries follow the theory, the back and forth retaliation will not happen between the two countries, and thus international trade will not end.

This answer choice gives us a reason for why countries close their markets to foreigners – to protect domestic producers.

Why countries close their markets to others has no impact on the argument. The argument is about what would happen IF every country reacted to a closed international market by closing some of their own to the other country. The reasons behind the closures are irrelevant.