所有的肽和蛋白质都能和硫酸铜的碱性溶液发生双缩尿反应
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He saw it as his d ________ to help the poor.
A. duty
B. dress
C. done
D. dry
Try to s________ every opportunity to practice English.
A. search
B. see
C. seek
D. son
For most students, studying a is a ________ wonderful experience.
A. along
B. alone
C. always
D. abroad
There’s a gun in pr ________ every American home.
A. practically
B. practical
C. poor
D. proof