根据句意提示补全单词。

1. May I use a ________ if I get lost in the forest.

2. The boy is going ________with his cousin if it is sunny tomorrow.

3. Riding on the roadway is not ________.

4. When you ride on the roadway, it is important to ________ the traffic rules.

5. You should walk your bike across the ________ and in school.

如图28所示，电源电压保持不变。只闭合开关S₁时，电压表V的示数为U₁，电阻R₁消耗的功率P₁=3W；只闭合开关S₂、S₃时，电压表V的示数为U₂，电流表A₁和A₂的示数分别为I₁和I₂；已知U₁:U₂ =1:3，I₁：I₂ =4:1，求：

（1）电阻R₂和R₃的比值；

（2）只闭合开关S₁时，电路消耗的总功率；

（3）通过开关的断开与闭合电路消耗的最大功率。