According to paragraph 3, which of the following is most important for an effective survey（）
A high number of respondents
B. Carefully worded questions
C. An interviewer's ability to measure respondents' feelings
D. A sociologist who is able to interpret the results
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(Heat) exhaustion is (causing) by excessive (loss) of body (fluids) and body salts.（）
A. Heat
B. causing
C. loss
D. fluids
According to the passage, the Mississippi River was（）
A. one of the boundaries of a region where new agricultural settlement took place
B. a major source of water for agricultural crops
C. the primary route by which agricultural crops were transported
D. a main source of power for most agricultural machinery
The word "prolific" in line 16 is closest in meaning to（）
A. fruitful
B. distinct
C. determined
D. energetic
The author mentions "wheal and wheat flour" in line 22 in order to（）
A. show that Americans exported more agricultural products than they imported
B. show the increase in the amount of wheat products exported
C. demonstrate file importance of cotton among American export products
D. demonstrate that wheat farming was becoming more profitable
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