Example 4. If $a>1$.
$$
\begin{array}{l}
p=\sqrt{1993 a}-\sqrt{1993 a-1}, \\
q=\sqrt{1993 a-1}-\sqrt{1993 a}, \\
r=\sqrt{1993 a}-\sqrt{1993 a+1}, \\
s=\sqrt{1993 a+1}-\sqrt{1993 a},
\end{array}
$$

Among $p, q, r, s$, the one with the smallest value is ( ).
(A) $p$
(B) $q$
(C) $r$
(D) $s$
Given $\because a>1$,
$$
\therefore p>0, s>0, q<0, r<0 \text {. }
$$

Therefore, we only need to determine the sizes of $\boldsymbol{y}, \boldsymbol{\%}$.
$$
\begin{array}{l}
\text { For } q_{2}=\frac{1}{\sqrt{1993 a-1}+\sqrt{1993 a}}, \\
r=\frac{-1}{\sqrt{1993 a}+\sqrt{1953 a+1}} . \\
\text { Also } 0<\sqrt{1993 a-1}+\sqrt{1993 a} \\
\quad<\sqrt{1993 a}+\sqrt{1993 a+1},
\end{array}
$$
$\therefore q<r$, i.e., $q$ is the smallest.