(Heat) exhaustion is (causing) by excessive (loss) of body (fluids) and body salts.（）
A. Heat
B. causing
C. loss
D. fluids
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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
The passage mentions all of the following aspects of plankton EXCEPT（）
A. the length of their lives
B. the level of the ocean at which they are found
C. their movement
D. their size
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