Sure, here are the answers to the questions:

11. She divorced him because of his **unkindness** to the children. (kind)
12. He has few friends because he is so **unsociable**. (society)
13. In **addition** to doing the cleaning, I make the coffee. (add)
14. Can we meet that **magician** at the party? (magic)
15. Please **acknowledge** our letter of the 10th. We have not had a reply. (know)
16. We are waiting for **the arrival** of his plane. (arrive)
17. It was a complete **failure** due to poor planning. (fail)
18. People who live in the cities spend their lives in an atmosphere of **continuous** noise. (continue)
19. At night we close our windows only to be kept **awake** by our neighbor's radio or television. (wake)
20. Doctors and **scientists** have reported on how we are all affected by the noise around us. (science)