Teacher:Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.
Teacher: Now, understand the problem? Solve this instance: Paragraph- Sent 1: Billy and Sally are brother and sister.
Sent 2: Billy is seven and Sally is eight.
Sent 3: Their mother, Deborah, likes to have Billy and Sally dress up in costumes and play a game where they are answering the telephone.
Sent 4: Usually when they play the game, Billy answers the telephone in a loud voice, and Sally answers the telephone in a quiet voice.
Sent 5: On Tuesdays, Billy answers in a quiet voice, and Sally answers in a loud voice.
Sent 6: On Fridays, Billy answers in a loud voice and Sally in a quiet voice.
Sent 7: Billy has blonde hair.
Sent 8: Sally has brown hair.
Sent 9: Deborah has blonde hair, and Billy and Sally's father, Bob, has brown hair.
Sent 10: He tells them to eat lettuce every time that he sees them, so that they grow big and strong like he is.
Sent 11: Deborah likes to add some sugar with the lettuce so that Billy and Sally know what it is like to have sweet tastes in their life.
Sent 12: One day, a Wednesday, Billy throws some lettuce into Sally's hair.
Sent 13: Deborah laughs an grabs some straw from their farm and puts it in Billy's hair.
Sent 14: Billy and Sally live on a farm.
Sent 15: They have a goat, named Joey, and a duck, named Quack.
Sent 16: They sometimes play a game with the goat where they chase him around the farm.
Sent 17: Other times, they play a game with Quack where they wave at Quack and laugh.
Sent 18: They have a fun life growing up on the farm. 
Question: Living on a farm, they have different pets, which are?.
Student:
Duck.