Разбираемся:
1 Is that the thief who had the red bag?
Yes, it is!
2 Are these the footprints which the Explorers followed?
No, they aren't!
3 Is this the bird who Pops interviewed?
Yes, it is!
4 Is this the house where the Explorers saw the brothers?
Yes, it is!
Проверка за 10 секунд: Заполнили пропуски местоимениями who, which, where в зависимости от вопроса.
Уровень эксперт: Местоимения who и which могут заменять союзные слова в сложноподчинённых предложениях, а where - союзное наречие.