13.2 The largest number among the following is
(A) 1 .
(B) $\sqrt{29}-\sqrt{21}$.
(C) $\frac{\pi}{3.142}$
(D) $5.1 \cdot \sqrt{0.0361}$.
(E) $\frac{6}{\sqrt{13}+\sqrt{7}}$.
(Mathematics Correspondence Competition for Junior High School, 1986, Some Provinces and Cities in China)
[Solution] $\because \sqrt{29}>5, \sqrt{21}>4, \quad \therefore \quad \sqrt{29}+\sqrt{21}>9$.
So $\sqrt{29}-\sqrt{21}=\frac{8}{\sqrt{29}+\sqrt{21}}20+2 \cdot 8=36=6^{2} \text {, } \\
\therefore \quad \frac{6}{\sqrt{13}+\sqrt{7}}<1 \text {, } \\
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Therefore, the answer is $(A)$.