6. The set $M=\{u \mid u=12 m+8 n+4 l$, where $m, n, l \in \mathbf{Z}\}, N=\{u \mid u=20 p+16 q+12 r$, where $p$, $q, r \in \mathbf{Z}\}$ has the relationship
(A) $M=N$
(B) $M \nsubseteq N, N \nsubseteq M$
(C) $M \subset N$
(D) $M \supset N$
6.【Analysis and Solution】For any element $u$ in $N$, we have
$$
u=20 p+16 q+12 r=12 r+8(2 q)+4(5 p) \in M \text{, }
$$

thus $N \subseteq M$.
On the other hand, for any element $u$ in $M$, we have
$$
u=12 m+8 n+4 l=20 n+16 l+12(m-n-l) \in \mathbf{N} \text{, }
$$

thus $M \subseteq N$, hence $M=N$.