下面C++代码用于交换两个正整数a和b的值，不能实现交换的代码是（ ）。

    cout << "输入第一个正整数: ";
    cin >> a;
    cout <<"输入第二个正整数: ";
    cin >> b;
    temp = a;
    a = b;
    b = temp;
    cout << "a=" << a << " b=" << b << endl;

    cout << "输入第一个正整数: ";
    cin >> a;
    cout <<"输入第二个正整数: ";
    cin >> b;
    b = a - b;
    a = a - b;
    b = a + b;
    cout << "a=" << a << " b=" << b << endl;

    cout << "输入第一个正整数: ";
    cin >> a;
    cout <<"输入第二个正整数: ";
    cin >> b;
    a = a + b;
    b = a - b;
    a = a - b;
    cout << "a=" << a << " b=" << b << endl;

    cout << "输入第一个正整数: ";
    cin >> a;
    cout <<"输入第二个正整数: ";
    cin >> b;
    a, b = b, a;
    cout << "a=" << a << " b=" << b << endl;