This possibility isn’t significant enough to affect how we interpret the study. As long as families that weren’t given a book ended up being less likely on average to own a self-help book than the ones that were given a book, the study’s results can still be evidence of cause.

The conclusion concerns the effect of “having a medical self-help book in the home.” Whether the families could access self-help through other means besides a book doesn’t relate to the effect of having a book in the home.

The flaw doesn’t concern one cause with multiple effects; it’s about one effect (fewer doc. visits) and two dcauses (book, improved family health). Also, if you view the books as causing two effects (improved health, fewer doc. visits), (C) is consistent with the reasoning.

The author overlooks that two different states of affairs (having book, improved family health) could each contribute to same effect (fewer doc. visits) even though neither contributes to the other. The books didn’t have to cause fewer visits through improving family health.

(E) might be true, but in the experiment 500 families were given a self-help book. So we know how these families got the book in their home. We then observed a decrease in doctor visits for these families after they were given a book.