ทำไม error ตรง clrscr();

#include "stdio.h"
#include "conio.h"
main()
{
    int a,b,c;
    float ans;
    clrscr();
    printf("number 1 =");
    scanf("%d",&a);
    printf("numder 2 =");
    scanf("%d",&b);
    printf("number 3 =");
    scanf("%d",&c);
    ans = (a+b+c)/3;
    printf("Average of %d, %d, %d = %f\n",a,b,c,ans);
}