You will be given a definition of a task first, then some input of the task.
In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: Sister Mary Bonaventure is in charge of the hospital ward of a convent in the county of Norfolk, England. She is troubled by her own sister's suicide, which she confides to her Mother Superior.
A torrential rain closes nearby roads, causing Sergeant Melling of the police to bring condemned murderer Valerie Carns there. She is being taken to prison.
Valerie was convicted of poisoning her brother Jason, a pianist. Jason's physician, Dr. Jeffreys, is head of the hospital where Sister Mary now works. Valerie still proclaims her innocence, but Jeffreys insists that she gave Jason a fatal overdose of his medicine.
A photograph of Jason clearly disturbs Isabel Jeffreys, the doctor's wife. He gives her a sedative. Valerie appeals to Sister Mary to bring her fiance, Sidney Kingham, to the convent to see her. A servant tells Sister Mary about the sadistic behavior of Jason Carns and produces a love letter to him, clearly written by Isabel.
Mother Superior is upset by Sister Mary's meddling. She burns the letter. The nun still intends to tell Melling the police sergeant what she knows.
Dr. Jeffreys is the one who gave Jason the fatal dose, and he might be slowly poisoning Isabel as well. He lures Sister Mary to a bell tower, where he attacks her. She rings the bell. Sidney hears it, rushes to her aid and overpowers Jeffreys, who is arrested by Melling.
Sister Mary's faith is restored, believing the rain that delivered Valerie to her to be divine intervention.
Output:
Who did Dr. Jefferys give a fatal dose of poison to?