题干

如图所示,沿水平方向的介质中的部分质点,每相邻两质点的距离相等,其中O为波源,设波源的振动周期为T,从波源通过平衡位置竖直向下振动开始计时,经 质点1开始振动,则下列关于质点的振动和介质中的波的说法中正确的是 (  )

A:介质中所有质点的起振方向都是竖直向下的,但图中质点9起振最晚

B:图中所画出的质点起振时间都是相同的,起振的位置和起振的方向是不同的

C:图中质点8的振动完全重复质点7的振动,只是质点8振动时通过平衡位置或最大位移处的时间总是比质点7通过相同的位置时落后

D:只要图中所有质点都已振动了,质点1与质点9的振动步调就完全一致,如果质点1发生的是第100次振动,则质点9发生的就是第98次振动

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A,C,D

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    Proudly reading my words, I glanced around the room, only to find my classmates bearing big smiles on their faces and tears in their eyes. Confused, I glanced toward my stone-faced teacher. Having no choice, I slowly raised the report I had slaved over, hoping to hide myself. “What could be causing everyone to act this way?”

    Quickly, I flashed back to the day Miss Lancelot gave me the task. This was the first real talk I received in my new school. It seemed simple: go on the Internet and find information about a man named George Washington. Since my idea of history came from an ancient teacher in my home country, I had never heard of that name before. As I searched the name of this fellow, it became evident that there were two people bearing the same name who looked completely different! One invented hundreds of uses for peanuts, while the other led some sort of army across America. I stared at the screen, wondering which one my teacher meant. I called my grandfather for a golden piece of advice; flip (掷) a coin. Heads—the commander, and tails—the peanuts guy. Ah! Tails, my report would be about the great man who invented peanut butter, George Washington Carver.

    Weeks later, standing before this unfriendly mass, I was totally lost. Oh well, I lowered the paper and sat down at my desk, burning to find out what I had done wrong. As a classmate began his report, it all became clear, “My report is on George Washington, the man who started the American Revolution.” The whole world became quite! How could I know that she meant that George Washington?

    Obviously, my grade was awful. Heartbroken but fearless, I decided to turn this around. I talked to Miss Lancelot, but she insisted: No re-dos; no new grade. I felt that the punishment was not justified, and I believed I deserved a second chance. Consequently, I threw myself heartily into my work for the rest of the school year. Ten months later, that chance unfolded as I found myself sitting in the headmaster's office with my grandfather, now having an entirely different conversation. I smiled and flashed back to the embarrassing moment at the beginning of the year as the headmaster informed me of my option to skip the sixth grade. Justice is sweet!