One cannot easily realize what a tremendous thing it is to know every trivial detail of twelve hundred miles of river and know it with absolute exactness. If you will take the longest street in New York and travel up and down it, conning its features patiently until you know every house and window and lamppost and big and little sign by heart, and know them so accurately that you can instantly name the one you are abreast of when you are set down at random in that street in the middle of an inky black night, you will then have a tolerable notion of the amount and the exactness of a pilot's knowledge who carries the Mississippi River in his head.

What is the effect of the word choice in the phrase “carries the Mississippi River in his head” in Paragraph 2 of the passage?

It shifts the tone in the paragraph from informative to humorous.

It emphasizes just how much the narrator believes a pilot knows.

It reveals that the phrase means the opposite of what it states.

It creates an absurd, nonsensical mood.