1. Given the set $M=\{2,0,1,9\}, A$ is a subset of $M$, and the sum of the elements in $A$ is a multiple of 3. Then the number of subsets $A$ that satisfy this condition is ( ).
(A) 8
(B) 7
(C) 6
(D) 5
- 1. B.

After enumeration, the subsets whose elements sum up to a multiple of 3 are $\{0\}, \{9\}, \{2,1\}, \{0,9\}, \{2,0,1\}, \{2,1,9\}, \{2,0,1,9\}$, totaling 7 subsets.