Why does the author mention corn and wheat yields in paragraph 7？
A. To illustrate the effect of a small amount of erosion
B. To explain why erosion affects only certain crops
C. To describe the result of a shift from traditional practices
D. To show how poor management influences crop yields
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Why does the author mention words, gestures, and pictures in paragraph 2？
A. To list things that are used in experiments with children
B. To give examples of symbols that children can understand
C. To compare different ways of illustrating a child's experience
D. To illustrate the concept of conservation of quantity
Traditional recruiting providers didn't make strategic progress because
A. they adopted new ways to help companies.
B. the solutions focused on project.
C. they didn't find customers.
D. they had no access to best practices and innovative techniques.
The phrase set up in paragraph 6 is closest in meaning to
A. forbid
B. establish
C. increase
D. conceal
Which sentence below best expresses the essential information in the highlighted sentence
Any species of life can be studied in population ecology．
B. Population ecologists care about the future of humanity．
C. The growth of the human population is a major concern．
D. Population ecology does not consider humans worthy of study．
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