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【单选题】
“ 实验四、重金属在土壤 — 植物体系中的迁移转化 ” ,该实验中利用原子吸收光谱法测定植物体内重金属 Pb 、 Cu 、 Cd 的含量时,需用对样品进行消解,本实验采用的是浓硝酸,实际湿式消解需要用到各种酸组成的混酸,以下哪种酸不合适作为混酸的成分?A.
硫酸
B.
盐酸
C.
氢氟酸
D.
硝酸
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【单选题】数字信号和模拟信号的特点是( )。
A.
模拟信号大小上、时间上均不连续，数字信号则相反
B.
数字信号大小上、时间上均不连续，模拟信号则相反
C.
数字信号、模拟信号大小上、时间上均不连续
D.
数字信号、模拟信号大小上、时间上均连续
【简答题】电子学研究的对象是载有信号的电信号,按其特点可以分为模拟信号和数字信号。模拟信号的特点是 ,数字信号的特点是 。
【单选题】They didn't watch the TV play that day, and______.
A.
so did we
B.
we didn't too
C.
so didn't we
D.
nor did we
【单选题】We used to have a discussion, ___?
A.
did you
B.
didn’t we
C.
used we
D.
didn't used we
【单选题】--Why didn't you answer the phone this afternoon?--We didn't hear it. I was in the garden and Mum ______ a shower.
A.
has had
B.
had
C.
was having
D.
had been having
【单选题】— You didn't wait for Tom yesterday. — Yes, but we ______. He didn't come. [ ]
A.
needn't have
B.
didn't need to
C.
could have
D.
should have
【简答题】We didn’t go fishing because it rained heavily．（改为同义句） We didn’t go fishing________ ________ the heavy rain．
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