SAT Text Structure and Purpose Question

The following text is from Jerome K. Jerome's 1889 novel Three Men in a Boat (To Say Nothing of the Dog). The narrator and two friends are taking a boat down the River Thames in England. In a boat, I have always noticed that it is the fixed idea of each member of the crew that he is doing everything. Harris's notion was, that it was he alone who had been working, and that both George and I had been imposing upon him. George, on the other hand, ridiculed the idea of Harris's having done anything more than eat and sleep, and had a cast-iron opinion that it was he -- George himself -- who had done all the labour worth speaking of. Which choice best describes the function of the underlined sentence in the text as a whole?