I could be mistaken, but I actually don't believe that (A) strengthens the argument at all. We are talking about two things here:
(1) reluctance to maintain quality of existing property
(2) reluctance to have additional rental-housing built
Now it
is true that (A) only talks about one while (B) talks about both. That may be enough of a case to pick (B) over (A). However, I don't think that (A) necessarily even
strengthens the first point but rather merely
discusses it. Let me explain.
In (A) we are simply saying that tenants prefer low-quality accommodations
with rent control to high-quality
without rent control. However, it is not like the landlord is going to
take away the rent control upon improving the quality of the housing! We would be much better off comparing two places of living that
both have rent control. In doing so, we could make the case that, yea, a landlord would be reluctant to maintain quality of existing property. Look at how much better of an answer this is...
Tenants prefer low-quality accommodations with rent control to high-quality accommodations also with rent-control
From this, we could more easily make the argument that maintaining the quality should be something that a landlord is reluctant to do.
In addition, understand that we are talking about
maintaining the quality of existing properties. Who is to say that those properties aren't incredibly high quality and not maintaining them would just make them marginally less so? We don't know anything about how high the quality of these places is so I would be hard-pressed to select an answer on information (low quality vs. high quality) that we simply do not know!
Even furthermore, I think when arguing for someone's position we should really refer to that someone. As mentioned, (A) focuses on the tenants when we are talking about the landlords. This is not to say that talking about the tenants should be an automatic elimination or something like that. It is merely to say that (B) is also better than (A) for that reason.