Answer:
B. The author wants to give readers a better understanding of the background and circumstances of transportation between the Atlantic and Pacific oceans.
Step-by-step explanation:
The introduction of a piece of text complementary to the main subject provides the reader with an insight about another topic (or in this case, a backgground), which offers valuable information related to the main idea without parting from it.
Regarding the example, the author most likely included this paragraph so it would serve as a counter-claim to a following chapter (referring to the placing of water dishes under the legs of beds).