My workbook converts "which really annoys him" to "it(this) annoys him" and I happened to remember there's little difference among "it", "this", and "that" in referring to the previous words or phrases.
So are the three frequently interchangeable or different from one another?
ex)Mr.Williams is a capable gardener. Last year, he came to work for Mrs.Elphinstone, who is old, fat, and wealthy. She knows nothing about gardens, but is under the impression that she know a lot, which really annoys him...