4. Given that $f$ is a mapping from set $M$ to set $N$, where $M=\{a, b, c\}, N=\{-3,-2,-1,0,1,2,3\}$, then the number of mappings $f$ that satisfy $f(a)+f(b)+f(c)=0$ is
A. 19
B. 30
C. 31
D. 37
Examine the case where the sum of three elements in $N$ is 0. Among these, the mappings $f$ corresponding to $(-3,1,2),(3,-1,-2),(-3,0,3),(-2,0,2)$, $(-1,0,1)$ each have 6, the mappings $f$ corresponding to $(-2,1,1),(2,-1,-1)$ each have 3, and the mapping $f$ corresponding to $(0,0,0)$ has 1. Therefore, the total number of mappings $f$ that satisfy the condition is 37. So the answer is $D$.