Subscription pricing
The question gives you an initial claim as well as a principle to go with it. The stem asks for you to pick an AC that could be appropriately used as a premise for an argument that uses the principle in the stimulus. I see this as more of a pseudo-weakening question because we're trying to supply a premise for a (non-existent) counter to the original claim.
Claim: County X lowered tariff barriers because it was good for foreign companies.
Principle: To confirm that someone advantaged from a change that was made, one must show how their interests played a role in bringing about said change.
I understand why AC 'E' is correct, but in what way does this question fall into the strengthening category?
I think this is ultimately a question that is up to preference and based on your current performance. For me personally, I like to really take my time during (and after BR) so I am confident in correcting the errors I made. I would rather take one test a week and understand every detail as opposed to taking two and not understanding the test as fluently.
That being said, some people are in a different stage in their preparation and are simply looking for repetition and practice. I would imagine these people are the one's who BR close to their target score and are only trying to fill in the gap between their timed performance and BR. Even in this instance though, careful and attentive review will only help your understanding and progress.
In short, if you think you just need practice/reps, then two tests a week might be a suitable option for you. If you have the time, however, one test a week would probably be more beneficial to learning than the former.