当浓度较高时进行原子发射光谱分析，其工作曲线(Ig~lgc)形状为()
A. 直线下部向上弯曲
B. 直线上部向下弯曲
C. 直线下部向下弯曲
D. 直线上部向上弯曲
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We’ll go swimming if the weather ____fine tomorrow.
A. is
B. was
C. will be
D. is going to be
It_____five years since he has left for Beijing.
A. was
B. has been
C. is
D. is going to be
Please don’t leave the office until your friend ___back.
A. came
B. comes
C. have come
D. will come
By the end of last year he___ about 1500 English words.
A. learns
B. learned
C. was learning
D. had learned
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