21. Look. An orange bird and a black bird (is /are) sitting in that tree. → 22. The economic and cultural centre of the United States (is/ are) New York. → 23. A car with poor brakes and no brake light (is/ are) dangerous. → 24. One of the girls who (has/have) gone to France lately. → 25. Much progress (has / have) been made by my students. → 26. The majority of students in my class (is / are) studying English. → 27. Two hours of skiing (provide / provides) a lot of exercise. → 28. Either of the dentists (is / are) available. → 29. We have got two cars, but neither of them (is / are) new. → 30. All the office staff (have / has) agreed to work late at night. →