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淡水资源紧缺的原因之一是人类的生产、生活造成了水的污染．某城镇生产、生活区的分布如下图所示．河流中A、B、C三处某次水样抽测结果如表所示． 项目 地点 A B C pH 6 4 5 1L水中溶解的氧气 0.011g 0.007g 0.003g 请根据上述信息回答下列问题： （1）为了减少硫酸厂废水对河水的污染，要选用一种廉价的物质进行处理，你选择的物质是______（填化学式）． （2）C处水草疯长，鱼虾几乎绝迹，主要原因是河水中______、P两种元素含量过高． （3）你认为在A、B、C三处中，最适合作自来水厂取水点的是______．
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举一反三
【单选题】Why do we need to design tasks to supplement the textbook?
A.
Textbooks are usually not well written
B.
Textbooks need adaptations to fit the needs of their target students
C.
Textbooks don ’ t have enough exercises
D.
Textbooks only cover a limited amount of language e skills
【单选题】But, to the extent that they lock us in to one way of looking at the world, we need to t ranscend them.
A.
keep away from
B.
go beyond
C.
see through
D.
be aware of
【单选题】Without facts,we cannot form a worthwhile opinion,for we need to have factual knowledge ______ our thinking.
A.
upon which to base
B.
which to be based on
C.
which to base on
D.
to which to be based on
【单选题】When we apply the t-test, we need to know that
A.
a t-test statistics can never be negative
B.
a t-test only applies to two-tailed test
C.
the degree of freedom is not that important when the number of observations is large
D.
a t-distribution is not identical to a standard normal distribution when the sample size goes to infinity
【单选题】We need to feel more for what we have instead of complaining what we don't.
A.
regret
B.
grateful
C.
offer
D.
break
【单选题】计算下图所示各机构的自由度,若有复合铰链、局部自由度或虚约束应明确指出,并判断机构的运动是否确定,(图中绘有箭头的构件为原动件)。
A.
有一个局部自由度，一个虚约束， F=3 × 4 － 2 × 4-1=1 因为自由度不等于原动件个数，所以没有确定的相对运动。
B.
有一个局部自由度，一个复合铰链， F=3 × 4 － 2 × 4-1=1 因为自由度不等于原动件个数，所以没有确定的相对运动。
C.
有一个局部自由度，一个虚约束， F=3 × 4 － 2 × 4-2=2 因为自由度等于原动件个数，所以有确定的相对运动。
D.
有一个虚约束， F=3 × 4 － 2 × 4-2=2 因为自由度等于原动件个数，所以有确定的相对运动。
【简答题】There are many things we need to know, but we do not learn them at school. For example, if we want to use our money wisely, we need to shop carefully. We need to know how to compare(比较) the prices of ...
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