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) $s$