根据短文内容，选择正确答案。
"Depend on yourself" is what nature says to every man. Parents can help you. Teachers can help you. Others still can help you. But all these people only help you to help yourself.
There have been many great men in history. Many of them were very poor when they were young, and had no uncles, aunts, or friends to help them. Schools were few and not very good. They could not depend on them for education. They tried their best to learn something and never gave up till they became well-known. One of the most famous teachers in England used to tell his pupils, "I cannot make worthy men of you, but I can help you make men of yourselves."
Some young men don't try their best to make themselves valuable to the human beings. If they see their weak points and change their ways, they can be successful. They are nothing now, and will be nothing as long as(只要) they live, unless they accept the advice of their parents and teachers, and depend on themselves.
的正弦交流电源上，变压器右侧部分为一火警报警系统原理图，其中R2为用半导体热敏材料制成的传感器，所有电表均为理想电表，电流表A2为值班室的显示器，显示通过R1的电流，电压表V2显示加在报警器上的电压（报警器未画出），R3为一定值电阻。当传感器R2所在处出现火情时，以下正确的是（ ）