3. The value of $(7+4 \sqrt{3})^{\frac{1}{2}}-(7-4 \sqrt{3})^{\frac{1}{2}}$ is ).
(A) $14 \sqrt{3}$
(B) $2 \sqrt{3}$
(C) $\sqrt{4}$
(D) $4 \sqrt{3}$
3. B

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