如何防止用户多次选择一个选项

我写了一个非常简单的问答c++程序(谁想成为百万富翁)，它从文件中读取问题，我想包括一个特殊的帮助选项，使用时会跳过答案。问题是，我不希望用户多次使用该选项，我该怎么做呢。

#include <iostream>
#include <fstream>
using namespace std;
int main()
{
string a[15]; //Array which will take answers
string print, print2; //Strings for printing questions
ifstream answers("q&a.txt");
cout << "Welcome to who wants to be a millionairen";
while (answers) //Read questions from the file
{
getline(answers, print);
auto position = print.find( "01." );
if (position <= print.size())
{
print2 = print.substr(position + 1 );
cout << print2 << endl << "A. Blackburn LosersnB. Blackburn RoversnC. Blackburn LoversnD. BlackburnWanderersn";
}
cin >> a[0];
if (a[0] == "b") //If correct, proceed to next question
{
getline(answers, print);
position = print.find("02.");
}
//If the answer is wrong, terminate the program
if (a[0] != "b")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. FridgenB. BanknC. MarketnD. Shoen";
}
cin >> a[1];
if (a[1] == "b")
{
getline(answers, print);
position = print.find("03");
}
if (a[1] != "b")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. BasseynB. KwesinC. AbiodunnD. Ejiman";
}
cin >> a[2];
if (a[2] == "b")
{
getline(answers, print);
position = print.find("04.");
}
if (a[2] != "b")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. InfernonB. DominonC. StilettonD. Tornadon";
}
cin >> a[3];
if (a[3] == "a")
{
getline(answers, print);
position = print.find("05.");
}
if (a[3] != "a")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. Marry a wifenB. Bury a dead parentnC. Have thanksgiving in churchnD. Accept gifts or favour in kindn";
}
cin >> a[4];
if (a[4] == "d")
{
getline(answers, print);
position = print.find("06.");
}
if (a[4] != "d")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. TenacitynB. VerifiabilitynC. HereditarynD. Validationn";
}
cin >> a[5];
if (a[5] == "c")
{
getline(answers, print);
position = print.find("07.");
}
if (a[5] != "c")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. ParisnB. CopenhagennC. New YorknD. Madridn";
}
cin >> a[6];
if (a[6] == "a")
{
getline(answers, print);
position = print.find("08");
}
if (a[6] != "a")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 =  print.substr(position +1);
cout << print2 << endl << "A. MumbainB. BeijingnC. Rio de JaneironD. Sevillen";
}
cin >> a[7];
if (a[7] == "b")
{
getline(answers, print);
position = print.find("09");
}
if (a[7] != "b")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. A speed equal to that of soundnB. A speed greater than that of soundnC. A speed equal to that of lightnD. A speed greater than that of lightn";
}
cin >> a[8];
if (a[8] == "b")
{
getline(answers, print);
position = print.find("10.");
}
if (a[8] != "b")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. NephronsnB. NervesnC. LigamentsnD. Stitchesn";
}
cin >> a[9];
if (a[9] == "c")
{
getline(answers, print);
position = print.find("11.");
}
if (a[9] != "c")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. SwimmernB. RefereenC. Football FannD. Judokan";
}
cin >> a[10];
if (a[10] == "b")
{
getline(answers, print);
position = print.find("12.");
}
if (a[10] != "b")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. FrancenB. United StatesnC. GermanynD. Indian";
}
cin >> a[11];
if (a[11] == "c")
{
getline(answers, print);
position = print.find("13.");
}
if (a[11] != "c")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. Clint EastwoodnB. Oliver StonenC. Peter JacksonnD. Morgan Freemann";
}
cin >> a[12];
if (a[12] == "a")
{
getline(answers, print);
position = print.find("14.");
}
if (a[12] != "a")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. LebanonnB. ColumbianC. JapannD. Eritrean";
}
cin >> a[13];
if (a[13] == "b")
{
getline(answers, print);
position = print.find("15.");
}
if (a[13] != "b")
{
cerr << "We are sorry, you are wrong!n";
break;
}
if (position <= print.size())
{
print2 = print.substr(position + 1);
cout << print2 << endl << "A. LiteraturenB. EconomicsnC. PeacenD. Medicinen";
cin >> a[14];
}
if (a[14] == "c")
{
cout << "Congratulations!nYou won a million dollars!n";
break;
}
if (a[14] != "c")
cerr << "We are sorry, you are wrong!n";
break;
}
return 0;
}

问题：

01. Which of these is the name of a British Football Club?
02. An establishment where money can be deposited or withdrawn is       called what?
03 Name given to a boy born on Sunday in Ghana is what?
04. Which of the following refer to the word fire?
05. According to the constitution a public officer is not allowed to do which of these?
06. The process by which genetic traits are transmitted from parents        to offspring is called what?
07. Roland Garros stadium is in which city?
08. Where is Tiananmen Square?
09. The word supersonic denotes which of these?
10. Which of these holds bones together at the joints of the body?
11. Linus Mbah achieved fame in Nigerian sporting circles as what?
12. DAX refers to the stock market of which country?
13. Who won the Academy Award for directing the movie ‘Million    Dollar Baby’?
14. In which country is the Galeras Volcano?
15. Professor Maathai Wangari won the Nobel Prize for which of these?

答案：

1 (B)
2 (B)
3 (B)
4 (A)
5 (D)
6 (C)
7 (A)
8 (B)
9 (B)
10 (C)
11 (B)
12 (C)
13 (A)
14 (B)
I15 (C)