选项
A.
A.$a^2(dfrac{1}{3}-dfrac{1}{2pi^2})$
B.
B.$dfrac{a^2}{2}(dfrac{1}{3}-dfrac{1}{2pi^2})$
C.
C.$a^2(dfrac{1}{3}-dfrac{1}{4pi^2})$
D.
D.$a^2(dfrac{1}{6}-dfrac{1}{2pi^2})$
正确答案：A
名词解析
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