36.68 There is a multiple-choice test consisting of six questions. The scoring criteria are: 8 points for a correct answer, 0 points for a wrong answer, and 2 points for no answer. A student scored 20 points in total, then he
(A) answered at most one question correctly.
(B) answered at least three questions correctly.
(C) left at least three questions unanswered.
(D) answered two questions incorrectly.
(2nd "Hope Cup" National Mathematics Invitational Competition, 1991)
[Solution] Let the student answer $x$ questions correctly, leave $y$ questions unanswered, and answer $z$ questions incorrectly. According to the problem,
$$
\left\{\begin{array}{l}
x+y+z=6, \\
8 x+0 y+2 z=20,
\end{array}\right.
$$

Here $x \geqslant 0, y \geqslant 0, z \geqslant 0$ and they are integers.
Solving the equations, we get $x=y=z=2$.
Therefore, the answer is $(D)$.