Sales of Fantasy books has gone from 1 to 10 percent of total Adult Fiction sales
+
# of favorable reviews of Fantasy books has increased
→
Increased sales of Fantasy books has increased because of more favorable reviews
TECHNIQUE
So this a fairly straightforward problem about correlation/causation. We have fantasy books increasing in terms of market share (more on that in a second) while number of favorable reviews are increasing "at the same time." The author concludes, "clearly the favorable reviews spawned the increased sales." How do we go about strengthening this claim? We could do it in a few ways...
- (1) Eliminate an obstacle to the conclusion by ruling out possible alternatives: an example of this would be something as weak as saying, "It wasn't better quality of books that spurred an increase in book sales"
(2) Show the same presumed cause, with the same effect: an example would be something like, "An increase of favorable reviews of nonfiction actually made an increase in sales of nonfiction."
(3) Show the absence of the presumed cause, with the absence of the effect: For example, "Book sales of sports biographies failed to increase when # of favorable reviews of sports biographies failed to increase as well."
Notice how all of these strengtheners can be fairly weak. We are not here to fix an argument.
ANSWER CHOICES
(A) While we might be able to make some crazy assumptions on why this would strengthen, it really doesn't because we don't know anything about the relationship between the reading level of book buyers and the reading level of these fiction books. This is simply out of scope.
(B) This provides an alternative to the conclusion; we want to get rid of an alternative to the conclusion. This is saying, "well maybe it wasn't the reviews but rather the happy endings that helped book sales!"
(C) This focuses on what happens after a book becomes popular. We want to focus on why the book genre became popular.
(D) This is very similar to (B). It provides an alternative explanation: maybe it was the advertising campaign instead of the favorable reviews that helped book sales.
(E) This strengthens! This says that after the reviews started to come out, book buyers began to take these books more seriously. While it doesn't actually say anything about causation necessarily, it does show that in one instance the favorable reviews came before the book sales, perhaps showing that the reviews actually do cause the book sales. This would fall most in line with strengthening technique #2.
***NOTE: This following point won't matter for this actual problem. However, I hope you noticed that we went from talking about market share to actual increased sales.