good question.
last line gives you enough hints to solve it.
read it once more and you can easily pick D as answer choice.
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Official Explanation

Conclusion: Thus, if current trends in news story writing and demand for advertising space continue, News Company's dependence on licensed, reprinted news stories is likely to decrease soon.

Premise: News Company…intends to decrease the amount of newspaper space devoted to news stories and increase the amount devoted to advertisements due to more advertisers who want to place ads in its newspaper than it is able to accommodate, and the number of news stories written by News Company's own staff is increasing steadily.

Assumptions: (1) It’s not a coincidence. The decrease must be due to a specific cause such as the amount of newspaper space. (2) There’s no other cause. The only things that affect space in the paper are the news stories and advertisements.

The phrase in the question stem most useful in evaluating, indicates that this is an evaluate question. The passage concludes reprinted news stories is likely to decrease soon because News Company…intends to decrease the amount of newspaper space devoted to news stories and increase the amount devoted to advertisements. As evidence, the passage cites the fact that there are more advertisers who want to place ads in its newspaper than it is able to accommodate, and the number of news stories written by News Company's own staff is increasing steadily. To evaluate the argument, the correct answer choice must convey information within the scope of the argument.

This passage contains a causality reasoning pattern because it states that the current trends in news story writing and demand for advertising space caused the decreased dependence on licensed, reprinted news stories. Because this is an evaluate question, the answers are in the form of questions. The correct answer will pose a question that if answered will assess the truth of the assumption. Information that rules out another cause will indicate that the assumption is true, which in turn will indicate the validity of the conclusion, while information that demonstrates another cause will indicate that the assumption is false, which in turn will indicate that lack of validity of the conclusion. Evaluate the answer choices.

Choice A: No. The question of When…will the amount of space filled by stories written by News Company's own staff exceed the amount of space filled by advertisements is out of scope because the conclusion mentions if current trends in news story writing and demand for advertising space continue, and the answer choice does not reference dependence on licensed, reprinted news stories.

Choice B: No. Other news organizations are out of scope. The argument discusses only News Company's newspaper.

Choice C: No. The mention of cartoons and other graphics is out of scope. The argument discusses only news stories and…advertisements.

Choice D: Correct. News Company’s plan to increase, maintain, or decrease the total amount of space available would help to determine the validity of the conclusion that News Company's dependence on licensed, reprinted news stories is likely to decrease soon. If the newspaper is going to stay the same size or get smaller, then the increase in space filled by advertisements as well as stories written by News Company's own staff will be enough to decrease the dependence on other sources. If the newspaper is going to get bigger, however, it might not.

Choice E: No. The mention of Sunday editions that have already been released is out of scope. The argument states News Company has recently announced that it intends to decrease the amount of newspaper space so changes that have already occurred would not help to evaluate the argument.

The correct answer is choice D.
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