33.28 $[x]$ represents the greatest integer not exceeding $x$, for example $\left[\frac{15}{4}\right]=3$. If
$$
y=4\left(\frac{x+[u]}{4}-\left[\frac{x+[u]}{4}\right]\right),
$$

and when
$$
\begin{array}{ll}
x=1,8,11,14 \text {, } & y=1 ; \\
x=2,5,12,15 \text {, } & y=2 ; \\
x=3,6,9,16 \text {, } & y=3 ; \\
x=4,7,10,13 \text {, } & y=0,
\end{array}
$$

then the expression for $u$ is
(A) $\frac{x+2}{4}$.
(B) $\frac{x+1}{4}$.
(C) $\frac{x}{4}$.
(D) $\frac{x-1}{4}$.
(China Junior High School Mathematics League, 1985)
[Solution] If $u=\frac{x+2}{4}$, then when $x=2$, $u=1,[u]=1$, thus $y=4\left(\frac{3}{4}-\left[\frac{3}{4}\right]\right)=3$, which contradicts the given condition that when $x=2$, $y=2$. Therefore, (A) is incorrect. Similarly, by setting $x=3$, we can determine that (B) is incorrect; by setting $x=4$, we can determine that (C) is incorrect. Hence, the correct choice is (D).