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问题 选择题

用下列实验或实验装置进行的实验中，能达到相应实验目的的是(   )

A．装置甲：防止铁钉生锈

B．测定同温同浓度的NaCl和NaF溶液的pH，确定F、Cl两元素的非金属性的强弱

C．装置乙：除去乙烷中混有的乙烯

D．用Ca(OH)2溶液来鉴别Na2CO3溶液和NaHCO3溶液

答案

答案：A

本题考查了化学实验的设计。元素非金属性的强弱可以通过元素的最高价氧化物对应水化物的酸性来比较，而通过比较无氧酸盐溶液的pH，无法确定元素的非金属性强弱，故B错；乙烯被酸性高锰酸钾溶液氧化，生成的CO2仍会混在乙烷中，故C错；D中Na2CO3溶液和NaHCO3溶液和Ca(OH)2反应都有CaCO3沉淀生成，故D错。

问答题 简答题

列举三个以上的元组关系演算中的集函数。
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问答题

Traditionally, the woman has held a low position in marriage partnerships. While her husband went his way, she had to wash, stitch and sew. Today the move is to liberate the woman, which may in the end strengthen the marriage union. Perhaps the greatest obstacle to friendship in marriage is the amount a couple usually sees of each other. Friendship in its usual sense is not tested by the strain of daily, year-long cohabitation. (46) Couples need to take up separate interests (and friendship) as well as mutually shared ones, if they are not to get used to the more attractive elements of each other’s personalities.
(47) Married couples are likely to exert themselves for guests--being amusing, discussing with passion and point--and then to fall into dull exhausted silence when the guests have gone. As in all friendship, a husband and wife must try to interest each other, and to spend sufficient time sharing absorbing activities to give-them continuing common interests, (48) But at the same time they must spend enough time on separate interests with separate people to preserve and develop their separate personalities and keep their relationship fresh.
(49) For too many highly intelligent working women, home represents chore obligations, because the husband only tolerates her work and does not participate in household chores. For too many highly intelligent working men, home represents dullness and complaints--from an over-dependent wife who will not gather courage to make their own life.
In such an atmosphere, the partners grow further and further apart, both love and liking disappearing. (50) For too many couples with children, the children are allowed to command all time and attention, allowing the couple no time to develop liking and friendship, as well as love, allotting exclusive parental roles.
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