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Must Be True. The correct answer choice is (C)
The stimulus in this Must Be True question contains a large number of facts concerning mushrooms’ ability to make beta-glucans. Pay careful attention to the language, as there are many rules involving conditional reasoning as well as rules involving Formal Logic. The first sentence states that mammals cannot digest cellulose and therefore cannot obtain glucose from wood. Mushrooms, on the other hand, can obtain glucose from wood and therefore can digest cellulose. Some mushrooms use this cellulose to make beta-glucans and some beta-glucan extracts from mushrooms cause antitumor activity in mammals by increasing immune cell activity. This antitumor activity caused by beta-glucans increases as the degree of polymer branching increases.
Answer Choice (A): This answer choice is too exaggerated to follow from the stimulus. While mammals may not be able to digest cellulose, they still might obtain beneficial health effects from cellulose.
Answer Choice (B): The stimulus provides enough information to determine that some extracts from mushrooms capable of using cellulose to make beta-glucans slow, reverse, or prevent the growth of cancerous tumors. As with any Formal Logical statement using “some,” this statement is reversible. That means we also know that some extracts that have these antitumor effects are from mushrooms that can use cellulose to make beta-glucans. However, the answer choice states that all extracts that have these antitumor effects must be from mushrooms with the ability to use cellulose to make beta-glucans. This answer choice exaggerates the logic in the stimulus and cannot be proven from the facts.
Answer Choice (C): This is the correct answer choice. We know from the last sentence that the antitumor activity of beta-glucans causes an increase in immune-cell activity. We also know this antitumor activity increases as the degree of branching increases. This allows us to conclude that the greater the degree of branching, the greater the degree of immune-cell activity triggered in mammals.
Answer Choice (D): The stimulus does not explain how immune-cells prevent tumor growth. The stimulus only explains how the beta-glucan extracts prevent tumor growth. While beta-glucan extracts increase immune cell activity instead of killing cancer cells directly, the immune cells could then directly target cancer cells and kill them.
Answer Choice (E): This answer choice is far too broad to be supported by the stimulus. While all mushrooms can obtain glucose from wood, only some mushrooms have the ability to make beta-glucans. There is not enough information here to conclude that all other organisms that can obtain glucose from wood share this ability to make beta-glucans.