1. **A number of books on U.S universities is in the library.**
- Mistake: "is" (D)
- Correction: "are"
- Explanation: The subject "A number of books" is plural, so the verb should be "are" instead of "is".

2. **I have created a small amount of videos that you can watch when you have the time.**
- Mistake: "amount" (B)
- Correction: "number"
- Explanation: "Amount" is used for uncountable nouns, while "number" is used for countable nouns like "videos".

3. **Polly has got a large number of hair cut off at the barber shop today.**
- Mistake: "number" (B)
- Correction: "amount"
- Explanation: "Hair" is an uncountable noun, so "amount" should be used instead of "number".