instruction:
In this task, you need to replace a letter in the sentence with another given letter.
question:
Sentence: 'a blue and white train is coming down some tracks'. Replace the letter 'b' with 'u' in the sentence.
answer:
a ulue and white train is coming down some tracks


question:
Sentence: 'a computer, keyboard, and mouse sitting on a desk'. Replace the letter 's' with 'c' in the sentence.
answer:
a computer, keyboard, and mouce citting on a deck


question:
Sentence: 'a horse on a city street with a building in the background'. Replace the letter 'r' with 'd' in the sentence.
answer:

a hodse on a city stdeet with a building in the backgdound