D for defining, and ND for non-defining. Decide where the relative pronoun can be omitted or not.
1. Buckingham Palace is in London. It is the residence of the Queen.
Buckingham Palace, ………
Buckingham Palace, which is in London, is the residence of the Queen of England.
ND – relative pronoun cannot be omitted
2. I bought a book yesterday. It’s very interesting.
The book ………
The book, (which) I bought yesterday, is very interesting.
ND –can be omitted
3. Sam’s birthday is on Saturday. He’s having a party this weekend.
Sam ………
Sam, whose birthday is on Saturday, is having a party this weekend.
ND –cannot be omitted