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【多选题】
Which of the following statement about cluster sampling is/are true?A.
Cluster sampling is used where the population is divided into already existing groupings.
B.
Clusters are usually formed by geographical locations.
C.
Cluster(s) is selected randomly from the population.
D.
In each selected cluster, simple random sampling or systematic sampling can be used for selecting sample members.
E.
In each selected cluster, even census can be used for selecting sample members.
F.
Cluster sampling can be viewed as a compromise between cost and representativeness, especially when research scales are large and travelling costs for fieldwork are high.
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【简答题】指出下面5个韵母的音节结构(即分别指出其韵头、韵腹、韵尾) uɑn ɑng iou ou ong
【多选题】下列关于自耦变压器减压起动的说法中正确的有( )。( 1 分)
A.
它是利用自耦变压器来降低起动时加在电动机定子绕组上的电压，以达到限制起动电流的目的
B.
起动时，定子绕组得到的电压是自耦变压器的二次电压
C.
一旦起动完毕，自耦变压器便被切除
D.
起动时，定子绕组得到的电压是自耦变压器的一次电压
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