c language, "" expression

"" Expression: binding from left to right, the last expression takes a value as a result

#include<stdio.h>
int main()
{
 int a = 3;
 int b = 3; 
 int c = 3;
 int d = 0;
 d = (a++, b++, a + b + c);
 printf("d = %d\n", d);
 return 0；