下面可以正确输出 They're planning a party for their friend's birthday. 的C++语句是？（ ）
cout << 'They\'re planning a party for their friend'\s birthday." << endl;
cout << "They\'re planning a party for their friend's birthday.'<< endl;
cout << 'They're planning a party for their friend's birthday.'<< endl;
cout << "They\'re planning a party for their friend\'s birthday." << endl;