Task: Question Generation
Input (English): Passage: Sonora Webster lives with her sister and abusive aunt during the Great Depression.  She learns that because of her accidentally letting the cows loose and her suspension from school, her treasured horse Lightning has been sold and she will be placed in an orphanage.  Instead, Sonora slips out of the house during the night. She ends up at a county fair and sees a performance by Marie, a diving girl who rides a horse off a platform, and aspires to do the same. Doc Carver, Marie's employer, tells her she is too young but gives Sonora a job as a stable hand due to her ability with horses, and she begins traveling with them. Doc's son Al wins a wild horse in a card game and Sonora names him Lightning. She later surprises Doc by taming and riding Lightning, so he promises to train her as a diving girl if she can mount it while it's moving, which she succeeds after multiple attempts.  
Marie falls and dislocates her shoulder, leaving her unable to perform, and Sonora steps in. Although she has never dived with Lightning, their first jump is successful.  Marie becomes jealous, and as Doc tires of her diva-like behavior, she quits rather than share billing with Sonora. Al develops a romance with Sonora that strains his relationship with his father, leaving after a particularly bad fight.  Al promises to write to Sonora, but Doc hides his letters.  As Doc and the new stable hand Clifford leave the farm in search of work, Lightning falls ill with colic. Al returns, and he and Sonora work together to heal Lightning. Doc fails to find any jobs, but Al announces he has arranged a six-month contract to perform at the Steel Pier in Atlantic City, New Jersey, reconciling father and son. Doc passes away en route from a heart attack, and Al assumes his father's role as show presenter.  Sonora searches for Doc's jacket to give Al confidence on his first show, and finds one of Al's letters inside, confessing his love for her, letting him know she feels the same.
Output (English): Whose letters did Doc Carver hide?