What does Orwell accomplish by including the phrase (I'm not exaggerating in paragraph 3?

A He adds humor to support his earlier descriptions of "Travelling" as a joke

B He introduces a new claim about coal-mining through his exaggeration and irony

C He addresses the possible counterargument that he is overemphasizing the difficulty of "Travelling"

D He provides supporting evidence for the idea that he is not suited for coal-mining