According to the latest study，most traffic accidents     be prevented if drivers are careful enough．

—Listen! Is Mary singing in the next room?—No. It _______ be Mary．She has gone to Shanghai for an important meeting.

Teenagers _______ allowed to drive in the street．

In general，she is devoted to her studies，but she________be little naughty at times.

― Why ______ you stay in the countryside, where it is not convenient to go shopping?―But I’ve found it rather valuable for my health.