Whether studying the output of the U.S. economy or how many classes a student will take, an unifying concept is that（）
A. wants are unlimited and resources are scarce, so trade-offs have to be made
B. both wants and resources are unlimited
C. wants are limited and resources are unlimited, so trade-offs have to be made
D. wants are limited and resources are unlimited, so trade-offs are unnecessary
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Which of the following is NOT the Na’vi’s attitude towards Nature（）
A. There is a web of connection among all things
B. Anything you get from nature will be returned
C. Natural resources can be exploited endlessly.
D. They worship nature as an equal and even higher power
Which of the following form of the English language is thought a free morpheme（）
A. The form non
B. The form un
C. The form nil
D. The form neg
Transposons are mobile genetic elements that jump from chromosome to chromosome by（）
A. homologous recombination
B. non-homologous recombination
C. site specific recombination
D. mutation
Which mutation will change a single codon so that one amino acid in a protein is replaced with a different amino acid（）
A. Silent mutations
B. Nonsense mutation
C. Missense mutation
D. Synonymous mutations
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