Read the excerpt from Jules Verne's Journey to the Centre of the Earth.

At the time that Harry and the professor embarked upon their journey, the authenticity of the jawbone's age was questioned by several people, including the M. Elie de Beaumont with whom the professor had several angry exchanges. This discovery essentially proves the professor correct. His entire impromptu lecture could be condensed into a quick "I was right" statement!
Using the context clues, what does "impromptu" mean?

something done in anger
completed without planning
nonfiction
done with a great deal of consideration