Hey John,
Thanks for the question. Let's break down the stimulus first:
- Premise: Businesses that used outdoor advertising (let's call them BOA) have larger market shares than those that did not.
Conclusion: Eliminating outdoor advertising would reduce the overall volume of business.
There are several errors in reasoning here. One, as you point out, is causal: the author assumes that outdoors advertising is at least one factor in increasing the market shares of those businesses that use it. The correlation between the two does not prove that one caused the other.
More importantly, we have a numbers-and-percentages error: the author is making a conclusion about the overall volume of business based on a shift in market shares. Even if we assume that outdoors advertising does indeed contribute to an increased market share of BOA, the author should have concluded that eliminating outdoor advertising will negatively affect BOA: perhaps their market share will indeed decrease. However, this does not mean that the
overall volume of business will decrease! Who is to say that the customers who formerly used BOA won't simply go elsewhere, and seek the services of a business that never used outdoor advertising before? The overall volume of business need not decrease; trade will simply be redistributed more equitably between the different business, as the BOA will no longer have a competitive advantage.
The author's assumption, therefore, is that the scenario I just described will NOT happen. In other words, he is assuming that BOA obtained a larger market share by some means other than diverting trade from competing businesses. If you're unconvinced, try the Assumption Negation Technique and see if the logical opposite of the assumption in (B) would weaken the argument:
Answer choice (B) when negated: The outdoor advertising increased market share solely by diverting trade from competing businesses.
If that were the case, then eliminating outdoor advertising will automatically re-divert this trade back to the competing businesses, and the overall volume of business will remain unchanged. Since the logical opposite of the assumption presented in answer choice (B) weakens the conclusion of the argument, answer choice (B) states the flawed assumption upon which the conclusion depends.
Hope this clears it up! Let me know if you have any questions
Thanks,