以下程序片段中，语法不正确的是（ ）。
int a, b; cin >> a + b; cout << a << "+" << b << '=' << a + b;
int a, b; cin >> a >> b; cout << a << "+" << b << '='; b = b + a; cout << b << endl;
int a, b; cin >> a >> b; int c = a + b; cout << a << "+" << b << '=' << c;
int a, b; cin >> a >> b; cout << a << "+" << b << '=' << a + b;
