During hot summer weather, how should the doors of a moving school bus be?

Answer: C — Closed but not locked

The doors of a moving school bus should be closed but not locked during hot summer weather. This is to ensure proper ventilation and to prevent overheating inside the bus. Opening the doors at all times (option D) is not safe as it may cause passengers to fall out or get injured. Keeping the doors closed and locked (option B) would also limit air flow, causing the bus to become stuffy and uncomfortable. Though the doors should be closed, they should not be locked as this would prevent the bus driver from quickly evacuating passengers in case of an emergency. Therefore, option C is the most appropriate choice.