Viết lại câu: Lan hasn't written to Maryam for a month

16. Lan hasn't written to Maryam for a month.
Lan last...............
17. The last time we surfed the Internet was 2 days
=>It is
18. We have gone swimming together since last week
=> It is
19. They haven’t taken their children to the zoo before
=> This is
20. Our children aren’t interested in eating ice-cream
=>Our children