Question

What provokes this response from Higgins?

"Of course I do, you little fool. Five minutes ago you were like a millstone round my neck. Now you're a tower of strength: a consort battleship. You and I and Pickering will be three old bachelors together instead of only two men and a silly girl."

Liza submits to his anger.
Liza realizes she doesn't need him anymore, and tells him so.
Liza fetches his slippers and offers to keep his appointments in order.
Liza threatens to assist Professor Nepean.