作业精灵老师示例：
1. Bobby and Tina go to see their aunt. They call Aunt Alice and ask the way
to her home. Alice says they can take the bus.
2. They get on the bus. There are so many cars going fast on the road.
3. There is a red light. The bus stops. The red light means they must stop.
4. The bus goes on, but soon it stops again. There aren't any traffic lights here.
But some elephants are crossing the road. They must stop and wait.
