What is the difference between c=a,b and c=(a,b)?

Question

What is the difference between the 4th and 5th lines?

int main() 
{
    int a = 3, b = 5, c, d; 
    c = a, b; 
    d = (a, b); 
    printf("c=%d d=%d", c, d); 
    return 0;
}

Output:

c=3 d=5