6. A unit ordered 22 boxes of fast food from a fast food store, spending a total of 140 yuan. There are three types of boxed meals: A, B, and C, with unit prices of 8 yuan, 5 yuan, and 3 yuan, respectively. Therefore, the number of different possible ordering schemes is ( . ) types.
(A) 1
(B) 2
(C) 3
(D) 4
6. C.

Let the unit order $x$, $y$, and $z$ boxes of meals A, B, and C, respectively. Then,
$$
\left\{\begin{array}{l}
x+y+z=22 \\
8 x+5 y+3 z=140 .
\end{array}\right.
$$
(1) $\times 8$ - (2) gives
$$
3 y+5 z=36 \Rightarrow 5 z=36-3 y \leqslant 36 \text {. }
$$

From this, we know $z \leqslant 7$, and $z$ is a multiple of 3, so the possible values of $z$ are $0, 3, 6$, with the corresponding $y$ values being $12, 7, 2$.
There are 3 sets of solutions:
$$
(x, y, z)=(10,12,0),(12,7,3),(14,2,6) .
$$