Teenagers growing pains

    Today, our students are facing many different problems and they want to ask some experts for help. Nancy says she feel stressed these days because of too much homework. She hasn't enough time to sleep for a long time and often falls asleep during the classes. John says he doesn't like to study because of the bad effect of computer games. Cindy says she gets fat because she likes to eat junk food and hardly ever takes exercise. Frank has the worst problem. He always fights with others as he doesn't know how to get on well with others and few of his classmates would like to talk with him.

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