平衡位置位于原点．的波源发出简谐横波在均匀介质中沿水平、轴传播，P、Q为x轴上的两个点（均位于x轴正向），P与O的距离为25cm，此距离介于一倍波长与二倍波长之间，已知波源自t=0时由平衡位置开始向上振动，周期T=1s．当波传到P点时，波源恰好处于波峰位置；此后再经过5s，平衡位置在Q处的质点第一次处于波峰位置，则P，Q之间的距离为（   ）

    "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.