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【单选题】
Students from a wide range of countries attend a professional exam: A questionnaire is sent to a selected number of students at the end of the exam which should be completed compulsorily. The samples are selected by randomly picking a number of students from each country. The sampling ratio of each country is the same as the proportion of sample size to population size.A.
Multi-stage
B.
Stratified
C.
Cluster
D.
Systematic
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