设集合 $A = \{x \mid x = 3k + 1, k \in \mathbf{Z}\}$,$B = \{x \mid x = 3k + 2, k \in \mathbf{Z}\}$,则( )
A. $A \cap B = \varnothing$
B. $A \cup B = \mathbf{Z}$
C. 若集合 $M = \{x \mid x = 6k + 2, k \in \mathbf{Z}\}$,则 $M \subseteq B$
D. 若集合 $N = \{x \mid x = 6k + 4, k \in \mathbf{Z}\}$,则 $N \subseteq A$