浅基础的类型根据结构形式可分为（）、（）、（）、（）、（）、和（）。
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The main idea of the second paragraph is______.
A. diversity of the workforce has its disadvantages
B. besides new opportunities, the changing workforce also contain challenges
C. the low educational levels of immigrants is a challenging problem
D. America had better face the problem of declining skills or it will suffer
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