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【多选题】
用ODS柱分析一有机弱酸混合物样品。以某一比例乙腈-流动相时，样品容量因子较小，若想使容量因子适当增加，如何改变流动相的组成( )A.
增加流动相中乙腈的比例
B.
增加流动相中水的比例
C.
增大流动相pH
D.
降低流动相pH
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【单选题】He has worked in the city _______ three years ago while his wife has been here______only one month.
A.
since , for
B.
since, since
C.
for, since
D.
for, since
【单选题】They have been skating ______. [ ]
A.
for three hours ago
B.
since three hours ago
C.
three hours ago
D.
since three hours
【单选题】以下说法错误的是
A.
微量元素在人体中的含量越多越好
B.
有的微量元素在人体中的含量过犹不及
C.
人体中的微量元素与环境保持动态平衡关系
D.
人体所需的微量元素主要来源于食物和水
【单选题】— When did the twins go to the aquarium? — _______. [ ]
A.
Three days ago.
B.
Next month.
C.
For four days.
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