In this excerpt from Mark Twain's "How to Tell a Story," which sentence is mostly clearly the thesis statement?
I do not claim that I can tell a story as it ought to be told. I only claim to know how a story ought to be told, for I have been almost daily in the company of the most expert story-tellers for many years.

There are several kinds of stories, but only one difficult kind—the humorous. I will talk mainly about that one. The humorous story is American, the comic story is English, the witty story is French. The humorous story depends for its effect upon the manner of the telling; the comic story and the witty story upon the matter.

The humorous story may be spun out to great length, and may wander around as much as it pleases, and arrive nowhere in particular; but the comic and witty stories must be brief and end with a point. The humorous story bubbles gently along, the others burst.

The humorous story is strictly a work of art—high and delicate art—and only an artist can tell it; but no art is necessary in telling the comic and the witty story; anybody can do it. The art of telling a humorous story—understand, I mean by word of mouth, not print—was created in America, and has remained at home.