­In the United States over the last quarter century, many services

­In the United States over the last quarter century, many services have been deregulated—controls regulating the prices charged to consumers have been removed. Deregulation has allowed providers to decide on their own pricing, but it has also allowed competition in provision of the services. Proponents of deregulation claimed that provided there was indeed competition, consumer prices would fall. They have been proved right, since in most cases prices did indeed fall once a service was deregulated.

The conclusion of the argument is the following:

They have been proved right

That conclusion does not include details but rather refers to "they" and what they "have been proved right" about. So, to fully understand the conclusion, we need to look back in the passage to make sure we understand what the conclusion refers to, which is the following:

Proponents of deregulation claimed that provided there was indeed competition, consumer prices would fall.

The support for the conclusion is the following:

in most cases prices did indeed fall once a service was deregulated

So, the reasoing of the argument is basically that, since prices fell once services were deregulated, the proponents of deregulation were correct in claiming that deregulation would cause prices to fall provided there was indeed competition.

Which of the following, if true, most strongly supports the argument?

The correct answer will strengthen the case for the conclusion that the proponents have been proved right.

A. After adjusting for inflation, prices for most services that have been deregulated fell significantly over the several years preceding their deregulation.

This choice weakens, rather than strengthens, the argument.

After all, if prices fell over several years preceding, i.e., before, deregulation, then maybe they just continued to fall after deregulatoin because of whatever had been causing to fall before. In that case, it would not make sense to conclude that the proponents were "proved right" in believing that deregulation would cause price decreases. After all, in that case, it would appear that deregulation is not actually the cause of the price decreases.

Eliminate.

B. For most regulated services, the regulations were intended to guard against the possibility that providers would use a monopoly position to dictate prices.

This choice has no clear effect on the strength of the argument.

After all, regardles of what the regulations were intended for, deregulation may or may not have resulted in price decreases.

Eliminate.

C. Surveys of consumers have shown that for many of the services that have been deregulated, consumers believe that the quality of service has fallen since deregulation.

This choice brings up another result of deregulation, quality of service falling.

While quality of service falling is likely related to deregulation, its relationship with prices isn't 100 percent clear. After all, quality of service could fall with or without a simultaneous decrease in price.

That said, a likely relationship is that quality of service falling is the result of price decreases. After all, decreases in price could squeeze companies, which would respond by reducing quality of service. In that case, we may have understand what caused quality of service to fall, but we don't have additional information on what caused prices to fall in the first place.

So, this choice has no clear effect on the argument.

Eliminate.

D. The few instances in which consumer prices increased after a service was deregulated were instances in which service providers faced little competition even after deregulation took effect.

This choice helps to prove the proponents of deregulation correct by showing that, only when a situation did not include both components the proponents said are needed, deregulation and competition, did prices not decrease.

In other words, it helps to strengthen the argument by showing that, essentially, there were no exceptions to their predicted outcome since the only times price decreases did not follow deregulation were times when there was little competition and their prediction is that deregulation would result in price decreases when there is indeed competition.

Keep.

E. For many deregulated services, providers who were operating both before and after deregulation became more profitable following deregulation.

This choice has no clear effect on the argument.

After all, the fact that providers became more profitable after deregulation doesn't clearly indicate that deregulation caused prices decreases. After all, companies that are "more profitable" may charge higher, lower, or the same prices.

Eliminate.

Correct answer: D