$$
\begin{array}{ll}
13 \cdot 15 \text { If } a>1, \text { and let } & \\
p=\sqrt{1993 a}-\sqrt{1993 a-1}, & q=\sqrt{1993 a-1}-\sqrt{1993 a}, \\
r=\sqrt{1993 a}-\sqrt{1993 a+1}, & s=\sqrt{1993 a+1}-\sqrt{1993 a} .
\end{array}
$$

Then the smallest value among $p, q, r, s$ is
(A) $p$.
(B) $q$.
(C) $r$.
(D) $s$.
(China Beijing Junior High School Mathematics Competition, 1993)
[Solution] From $p-q=2 \sqrt{1993 a}-2 \sqrt{1993 a-1}>0$, we know $p>q$; also $r-s=2 \sqrt{1993 a}-2 \sqrt{1993 a+1}<0$, we know $\quad r<s$. Thus, $\quad p>q, s>r>q$, hence the answer is $(B)$.