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
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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
在对人的正常心理与异常心理进行区分时，统计学标准的局限性是( )。
A. 远离平均数的两端被视为异常
B. 多以心理测验法为工具，获得确定正常与异常的界限
C. 有些心理特征和行为不一定成常态分布
D. 位于平均值的大多数人属于心理正常范围
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