A:That's the playground. We can play football there.
B:That's the classroom. We can have lessons there.
小明设计了一种照明电路图,其设计要求是:S1、S2分别为楼上和楼下的开关(都是单刀双掷开关),要求拨动其中任一开关,都能改变电灯原来的发光或熄灭状态.图所示的四幅电路图中,既符合上述设计要求,又符合安全用电要求的是
The car accident happened ____ a cold winter morning. Luckily, no one was hurt.
下列有关某湖泊生态系统描述正确的是