Ld returned 1 exit status in C programming

#include <stdio.h>
#include <math.h>


int menu (void);
int calBMI(int [], double [][2], int,double []);
int category(double);
double  avgBMI(double [],int);
int avgCategory(double);


int main (void)
{
const int numOfStudents=10, SIZE=25, NUM=2;
char students[numOfStudents][SIZE];
int age[numOfStudents];
double weightHeight[numOfStudents][NUM];
double BMI[numOfStudents];
int underweight[numOfStudents];
int normalweight[numOfStudents];
int overweight[numOfStudents];
int obese[numOfStudents];
int i,j,a=0,b=0,c=0,d=0,count=0,option;
double averageBMI;

FILE*input, *output;    // create two files
input= fopen( "in.dat", "r");
output= fopen( "out.out", "w");

 for(i=0; !feof(input)&&i<numOfStudents; i++)    // !feof means not end of file
 {
    fgets(students[i], 25, input);
    fscanf(input, "%d\t", &age[i]);
    for(j=0; j<NUM; j++)
    {
        fscanf(input, "%lf\t", &weightHeight[i][j]);
    }
 }

  calBMI(age, weightHeight, numOfStudents, BMI);

 while(count<numOfStudents)
 {
    if(BMI[count]<18.5)
    {
        underweight[a]=count;
        a++;
    }
    else if(BMI[count]<25)
    {
        normalweight[b]=count;
        b++;
    }
    else if(BMI[count]<30)
    {
        overweight[c]=count;
        c++;
    }
    else
    {
        obese[d]=count;
        d++;
    }
    count++;
 }
 // for user to choose menu
menu();
scanf( "%d", &option); // read the integer value of option

switch(option)
{
    case 1: printf( "\nList of students\n" );
            for(count=0; count<a; count++)
            {
                printf( "%d. %s\n", count+1, students[underweight[count]]);
            }
            break;
    case 2:  printf( "\nList of students\n" );
             for(count=0; count<b; count++)
             {
                printf( "%d. %s\n", count+1, students[normalweight[count]]);
             }
             break;
    case 3:  printf( "\nList of students\n" );
             for(count=0; count<c; count++)
             {
                printf( "%d. %s\n", count+1, students[overweight[count]]);
             }
             break;
    default: printf( "\nList of students\n" );
             for(count=0; count<d; count++)
             {
                printf( "%d. %s\n", count+1, students[obese[count]]);
             }
             break;
   }


    printf( "\nThe number of students for category underweight is %d\n", a);
    printf( "The number of students for category normalweight is %d\n", b);
    printf( "The number of students for category overweight is %d\n", c);
   printf( "The number of students for category obese is %d\n", d);


   averageBMI=avgBMI(BMI, numOfStudents);
   printf( "\nThe average BMI of all students is %.2f\n", averageBMI);
   avgCategory(averageBMI);

   fprintf(output, "Name\t\t\tAge\tweight(kg)\theight(m)\tBMI\tCategory\n");
   fprintf(output, "==========================\t====\t=======\t======\t======\t========\n");
   fclose(output);

   for(count=0; count<numOfStudents; count++)
   {
    output = fopen( "out.out", "a");
    fprintf(output, "%s", students[count]);
    fprintf(output, "%d\t", age[count]);
    fprintf(output, "%.2f\t\t", weightHeight[count][0]);
    fprintf(output, "%.1f\t\t", weightHeight[count][1]);
    fprintf(output, "%.2f\t", BMI[count]);
    fclose(output);
    category(BMI[count]);
   }


   output = fopen( "out.out", "a");
   fprintf(output, "\nThe number of students for category underweight is %d\n", a);
   fprintf(output, "The number of students for category normalweight is %d\n", b);
   fprintf(output, "The number of students for category overweight is %d\n", c);
   fprintf(output, "The number of students for category obese is %d\n", d);

   // display the average BMI of all students and average category
   fprintf(output, "\nThe average BMI of all students is %.2f\n", averageBMI);
   fprintf(output, "The average category is ");
   fclose(output);
   category(averageBMI);

   fclose(input);
   fclose(output);
   return 0;
   }


int calBMI(int age[], double weightHeight[10][2], int size, double BMI[])
{
int index=0;
do
{
    if(age[index]<=18)
        BMI[index]=(weightHeight[index][1]/pow(weightHeight[index][0]/100,2)*1.2);
    else
        BMI[index]=(weightHeight[index][1]/pow(weightHeight[index][0]/100,2));
    index++;
  } while(index<size);
   return 0;
  }


 int menu(void)
 {
  printf( "\t\t1. Underweight\n");
 printf( "\t\t2. Normalweight\n");
 printf( "\t\t3. Overweight\n");
 printf( "\t\t4. Obese\n");
 printf( "Please enter the category >> ");
 return 0;
  }


  double avgBMI(double BMI[], int size)
 {
int index;
double sum=0;

for(index=0; index<size; index++)
{
    sum+=BMI[index];
}
return(sum/size);
}


int avgCategory(double averageBMI)
{
if(averageBMI>0)
{
    if(averageBMI<18.5)
        printf( "The average category is underweight\n");
    else if(averageBMI<25)
        printf( "The average category is normalweight\n");
    else if(averageBMI<30)
        printf( "The average category is overweight\n");
    else
        printf( "The average category is obese\n");
}
else
    printf( "Invalid\n");
return 0;
}


int category(double BMI)
{
int index;
FILE*output;
output = fopen( "out.out", "a");

if(BMI>0)
{
    if(BMI<18.5)
        printf( "underweight\n");
    else if(BMI<25)
        printf( "normalweight\n");
    else if(BMI<30)
        printf( "overweight\n");
    else
        printf( "obese\n");
}
else
printf( "Invalid\n");
return 0;
}

Do anyone have any idea regarding the error?
    The error ld returned 1 exit status and i've already check that there is no old 
     instance of program is still running.