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单项选择题

I arrived at six o’clock, but Mary ______ already ______.
A、have … gone
B、has … gone
C、had … gone

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单项选择题What is repetition?

A.The use of the same letter or sound at the beginning of words.
B.Speaking or writing that makes something sound better, more exciting, etc. than it really is.
C.Comparing something to something else.
D.Doing or saying the same thing many times.

单项选择题‏一束光具有两条强度相等的谱线,且每条谱线都是线偏振光,但合成光的偏振度却近似为0,导致这个结果的原因是( )。‌

A.两条线偏振谱线的偏振方向相同
B.两个线偏振谱线的偏振方向成45度角
C.两个线偏振谱线的偏振方向成60度角
D.两个线偏振谱线的偏振方向垂直