问题
单项选择题
机械波在媒质传播过程中，当一媒质质元的振动动能的相位是π／2时，它的弹性势能的相位是：（）
A.1/2π
B.π
C.2π
D.无法确定
答案
参考答案：A
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机械波在媒质传播过程中，当一媒质质元的振动动能的相位是π／2时，它的弹性势能的相位是：（）
A.1/2π
B.π
C.2π
D.无法确定
参考答案：A
I grew up in a small town. When I was in the third grade, I was chosen to be the in the school play. My mother helped me practice my lines for weeks. But once I stepped onto the stage, I was too nervous to the words. Finally, my teacher asked me to the role to a storyteller. I didn’t tell my mother had happened when I went home for lunch that day. She knew that I was from my look. She asked if I wanted to go for a walk in the yard. It was a lovely day and the grass was turning green. We could see a lot of yellow dandelions(蒲公英）in the grass. I watched my mother walking towards a dandelion and it up by its roots(根茎). “From now on, we’ll have only roses in this garden.” she said. “But I like dandelions,” I said. “All are beautiful —— even dandelions!” “Yes, every flower gives pleasure in its own way. And that is true of too. Not everyone can be a princess. You will be a beautiful storyteller,” my mother said, reminding me of how much I to read stories to her. “The storyteller’s part is just as as the part of a princess.” The big day came. A few minutes before the play, my teacher handed me a . I looked at it and thought of our talk. It made me . After the play, I took the flower home, and kept it in our family dictionary.
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