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答案：B
解析：
Choice B is correct. When multiplying polynomials, be sure to multiply each term in the first polynomial by each term in the second polynomial.
This can be done as follows: (a+3)(2a^2-5a+7)=2a^3-5a^2+7a+6a^2-15a+21
Combine like terms to get:
2a^3+a^2-8a+21
The equivalent expression is 2a^3+a^2-8a+21.
