Everything seems to be Ok, but wa3! Josepth algo works correctly (to my mind).
Maybe i must skip not only '\n', but something else.
Here is source:

#include<stdio.h>
char str[30010];
int len=0;
int main()
{
    while(!feof(stdin))
    {
        scanf("%c",&str[len]);
        if(str[len]!='\n')len++;
    }
    int res=0;
    for(int i=1;i<=len-1;i++) res=(res+1999)%i;
    if(str[res]=='?') printf("Yes");
    else
    {
        if(str[res]==' ') printf("No");
        else printf("No comments");
    }
    return 0;
}