看不懂答案为什么是78？

Question

#include<stdio.h>int main(){ int i,j; int a[3][4]={1,2,3,4,5,6,7,8,9,10,11,12}; int sum=0; for(i=0;i<3;i++) for(j=0;j<4;j++) sum+=a[i][j]; printf("sum=%d\n",sum); return 0;}