C++ program

I have some code that is meant to prompt the user to enter 10 scores and choose one of three structures(if, while, or do-while) to then evaluate and display the name, score(%), and letter grade.  This is to be done using call by value.  I have spent hours on this and keep getting an external linker error.  What am I doing wrong?  Do I need all of these libraries?

#include <iostream.h>
#include <conio.h>
#include<ctype.h>
#include<iomanip.h>
#include<stdlib.h>
#include<math.h>

int Prompt_User_For_Selection(char);
int Service_Selection_Using_IfElse(char);
int Determine_LtrGrd_Using_Switch(float);


int main()
{
 char x;
 clrscr();                 //clear screen
 Prompt_User_For_Selection(x);
}
int Prompt_User_For_Selection(char start)
{
  char ans,y;
  do{
  clrscr();
  cout<<"STRUCTURE               SELECTION\n";
  cout<<"\n";
  cout<<"While                       W\n";
  cout<<"DO While                    D\n";
  cout<<"For                         F\n";
  cout<<"\n";
  cout<<"Enter Selection <Q to Quit>:\n";
  cout<<"\n\n\t";
  getch();
  cin>>ans;
  ans=toupper(ans);
  }while(ans!='W'&& ans!='D'&& ans!='F'&& ans!='Q');
   if (ans!='Q')
     return 0;
   else
    clrscr();
    cout<<"\n\n\tYou Selected to Quit BYE";
    cout<<"\n\n\tHit any key to exit:";
  getch();
  ans=start;
  y=Service_Selection_Using_IfElse(ans);
  return(y);
}
int Service_Selection_Using_IfElse(char ans2)
{
     char choice;
     int  final;
     int Execute_While_Loop();
     int Execute_Do_While_Loop();
     int Execute_For_Loop();


     char firstname[20];
     cout<<"Enter Your First Name:\n";
     cin>>firstname;
     char lastname[20];
     cout<<"Enter Your Last Name:\n";
     cin>>lastname;

     clrscr();

     if(choice=='W'){
      cout<<"\n\n\tYou Selected the While Structure\n";
      choice=ans2;
      Execute_While_Loop();

     }
     else
       if(choice=='D'){
        cout<<"\n\n\tYou Selected the Do While Structure\n";
        choice=ans2;
        Execute_Do_While_Loop();
       }
       else
        if(choice=='F'){
         cout<<"\n\n\tYou Selected the For Structure\n";
         choice=ans2;
         Execute_For_Loop();
        }

     cout<<"\n\n\n\n\tStudent Name:\t"<<firstname<<' '<<lastname<<endl<<endl;
     cout<<"\tGrade Average:\t"<<final<<"%"<<endl;
     final=Determine_LtrGrd_Using_Switch(choice);
     return(final);
     cout<<"\n\n\tHit any key to continue:";
     getch();
}
int Execute_While_Loop(float avg)
{
       int zz,result;
       int total,
           scoreCounter,
           score;
       total=0;
       scoreCounter=1;

       cout<<"\n\n\tEnter Score number"<<' '<<scoreCounter<<":\t";
       cin>>score;
       total=total+score;

       while(scoreCounter<10){
        while(score>100||score<0){
         cout <<"\n\n\tEnter Score number"<<' '<<scoreCounter<<":\t";
         cin>>score;
         }
        total=total+score;
        ++scoreCounter;
        cout<<"\n\n\tEnter Score number"<<' '<<scoreCounter<<":\t";
        cin>>score;
       }

       clrscr();
       zz=static_cast<float>(total)/scoreCounter;
       zz=avg;
       result=Determine_LtrGrd_Using_Switch(zz);
       return (result);
}
int Execute_Do_While_Loop(float avg2)
{
   int qq,result2;
   int total,
       scoreCounter,
       score;
   total=0;
   scoreCounter=1;

   cout<<"\n\n\tEnter Score number"<<' '<<scoreCounter<<":\t";
   cin>>score;
   total=total+score;
   do{
   cout <<"\n\n\tEnter Score number"<<' '<<scoreCounter<<":\t";
   cin>>score;
    do{
    total=total+score;
    ++scoreCounter;
    cout<<"\n\n\tEnter Score number"<<' '<<scoreCounter<<":\t";
    cin>>score;
    }while(scoreCounter<10&&score>=0&&score<=100);
   }while(score>100||score<0);
   clrscr();

   qq=static_cast<float>(total)/scoreCounter;
   qq=avg2;
   result2=Determine_LtrGrd_Using_Switch(qq);
   return result2;
}
int Execute_For_Loop(float avg3)
{
   int vv,result3;
   int total,
      score,
      scoreCounter;
   for(int total=0,scoreCounter=1;scoreCounter<=10;++scoreCounter,
        total=total+score){
      for(int scoreCounter;0>score||score>100;scoreCounter){
        cout<<"\n\n\tEnter Score number"<<' '<<scoreCounter<<":\t";
        cin>>score;
     }
      cout<<"\n\n\tEnter Score number"<<' '<<scoreCounter<<":\t";
      cin>>score;
   }

    clrscr();
    vv=static_cast<float>(total)/scoreCounter;
    vv=avg3;
    result3=Determine_LtrGrd_Using_Switch(vv);
    return(vv);
}
int Determine_LtrGrd_Using_Switch(float results)
{   int last;
    char LtrGrd;

    LtrGrd=floor(last*10+.5)/100;

    switch(LtrGrd){
      case 10:
      case 9:
       cout<<"\n\tLetter Grade:\t"<<"A";
       break;
      case 8:
       cout<<"\n\tLetter Grade:\t"<<"B";
       break;
      case 7:
       cout<<"\n\tLetter Grade:\t"<<"C";
       break;
      case 6:
       cout<<"\n\tLetter Grade:\t"<<"D";
       break;
      case 5:
       cout<<"\n\tLetter Grade:\t"<<"F";
       break;
    }
      last=results;
      return 0;
}