1. Set
$$
A=\{1,3,5,7\}, B=\{2,4,6,8,20\} \text {. }
$$

If $C=\{s \mid s=a+b, a \in A, b \in B\}$, then the number of elements in set $C$ is ( ).
(A) 9
(B) 11
(C) 13
(D) 20
-1. B.
Note that $s$ is odd, with a minimum value of 3 and a maximum value of 27, and it is missing 17 and 19. Therefore, set $C$ has a total of 11 elements.