One of the following sentences is nonsensical. Which one is it?
Options:
- Sentence A: "I was riding on the back of a friend's while"
- Sentence B: "I was riding on the back of a friend's bicycle"
 Let's be accurate as possible. So think first.

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
A while isn't a thing that can be ridden on.
The final answer: Sentence A.