1. (1989 National High School Mathematics League Question) The relationship between the sets $M=\{u \mid u=12 m+8 n+4 l, m, n, l \in \mathbf{Z}\}$ and $N=$ $\{u \mid u=20 p+16 q+12 r, p, q, r \in \mathbf{Z}\}$ is ( )
A. $M=N$
B. $M \nsubseteq N, N \nsubseteq M$
C. $M \subset N$
D. $M \supset N$
1. Start from the expressions of elements in the two sets.

Since $12 m+8 n+4 l=4(3 m+2 n+l)$,
$$
20 p+16 q+12 r=4(5 p+4 q+3 r),
$$

and $(3,2,1)=1,(5,4,3)=1$, by Bézout's theorem, we know that
$3 m+2 n+l$ and $5 p+4 q+3 r$ can both represent all integers, so, $M=N=\{k \mid k=4 l, l \in \mathbf{Z}\}$. Therefore, the answer is A.