PROBLEM WITH for loop & structure

#include <stdio.h>
typedef struct {
				  char name [20];
				  int ID,year;
				  char major [4];
				  float GPA;
	     }data;


/************************* Main FUNCTION ********************************/
int id,num=0;
int i,d;
data a[100];

for(i=0;i<100;i++)// error  wHY ?? 
     a[i].ID =0;


// Error WHy? 
do {
  printf(" \n Welcome in the student data base \n\n");
  printf("\n\t\t\t 1. Add new student ");
  printf("\n\t\t\t 2. Update  student ");
  printf("\n\t\t\t 3. Display student ");
  printf("\n\t\t\t 4. Quit            ");

  printf("\tWhat are you like to do choice (1-4) :  ");
  scanf (" %d", &d);

  }while (d!=5 && num <31);