Fact 1: the Labrador has been most popular breed of dog in the United States for several years.
Fact 2: Labradors are friendly and easily trained

Conclusion: the popularity is only because of the friendly disposition and trainability

We need to look for an answer that shows that these may not be the ONLY reasons, that there must be other reasons.

Answer Choices:
A. The Labrador Retriever is closely related to the Golden Retriever, another popular breed in the United States.
So what? Irrelevant

B. Labrador Retrievers have been trained in a number of useful ways, such as to serve as guide dogs or as hunting companions.
We cannot assume that usefulness is related to popularity.

C. Some people do not like Labrador Retrievers because they can be quite large as adults and often chew furniture when they are puppies.
Irrelevant

D. The Australian Shepherd is another friendly and easy-to-train dog breed, yet it does not have nearly the popularity of the Labrador Retriever.
This works: if friendliness and trainability made breeds popular, then the Australian Shepherd would be as popular. The fact that the Australian Shepherd is not as popular shows that there must be some other factors that contribute to the Labrador's popularity.

E. No recent popular television shows have featured a Labrador Retriever.
Irrelevant