为什么m[1] - m[0]返回3而m是3x3矩阵?

This is my code:

这是我的代码:

int m[][3] = {
               { 0 , 1 , 2  },
               { 10, 11, 12 },
               { 20, 21, 22 }
             };
printf("%d %d\n", m[1] - m[0], m[1][0] - m[0][0]);
int m[][3] = {