- Differential files are small and localized, which makes it/these files inexpensive to back up regularly.
- "which" refers to the sentence before it. It's not the files which make back-up inexpensive, it's the fact that the files are small and localized which makes it inexpensive.
- I created two tables that are used by the IT departments.
- BETTER: ...that the IT department uses.
Student or Learner