按要求改写句子，每空一词。

1. The man outside the door must be Mr Green. (改为否定句)

    The man outside the door _____ _____ Mr Green.

2. This could be Robert's basketball. (对划线部分提问)

    _____basketball_____this be?

3. Do you know whose the book is? (改为同义句)

    Do you know_____the book_____?

4. There are some children who are having fun in the yard. (改为同义句)

    There are some children_____in the yard.

5. She could be running for exercise. (对划线部分提问)

    _____she_____running for?