How does Cathy obtain Nelly's letter in Chapter 31? What does she tell Mr. Lockwood regarding a response?

Mr. Lockwood discreetly drops a letter from Mrs. Dean in Catherine's lap, but Hareton notices the exchange, and takes the letter to show Heathcliff. When Catherine begins to cry, her cousin relents and throws the letter at her. Cathy reads the letter, but regrets she cannot answer it, as she has no paper and no books.