1. Nothing can live without air. (变为同义句)
    Nothing can live if __________ is _________ air.
2. It's about 20 minutes' walk to the farthest restaurant. (就划线部分提问)
    _________ _________ is it to the farthest restaurant?
3. The little girl is too young to stay at home by herself. (变为同义句)
    The little girl ________ old ________ to stay at home _______.
    The little girl is ________ young ________ she can't stay at home by herself.
4. I think math is the most difficult subject of all. (变为同义句)
    I think math is ________ ________ than the ________ subjects.
5. English is more interesting than history. (变为同义句)
    History is _________ interesting ________ English.
    History isn't so _________ as English.