$1 \cdot 72(7+4 \sqrt{3})^{\frac{1}{2}}-(7-4 \sqrt{3})^{\frac{1}{2}}$ 的值为  
(A) $14 \sqrt{3}$.  
(B) $2 \sqrt{3}$.  
(C) $\sqrt{4}$.  
(D) $4 \sqrt{3}$.  
(China Sichuan Province Junior High School Mathematics Competition, 1992)  

The value of $1 \cdot 72(7+4 \sqrt{3})^{\frac{1}{2}}-(7-4 \sqrt{3})^{\frac{1}{2}}$ is  
(A) $14 \sqrt{3}$.  
(B) $2 \sqrt{3}$.  
(C) $\sqrt{4}$.  
(D) $4 \sqrt{3}$.  
(China Sichuan Province Junior High School Mathematics Competition, 1992)
[Solution] The original expression $=\sqrt{(2+\sqrt{3})^{2}}-\sqrt{(2-\sqrt{3})^{2}}=2 \sqrt{3}$. Therefore, the correct choice is $(B)$.