Below is my code. When I submit it in UVA they are giving me Presentation error. I want to know the reason.

I have tried all possible ways. In my code I use 2D-array to store each word of a sentence. I also want to know the reason for giving presentation error.

#include<stdio.h>
#include<string.h>
int main()
{
    char a[10000],ar[100][10000];
    int i,j,k,m,n,l,len[10000],c;
    freopen("input.txt","r",stdin);
    while(gets(a))
    {
        k=0;
        i=0;
        c=0;
        for(j=0; ; ++k)
        {
            if(a[k]==' ' || a[k]=='\0')
            {
                ar[i][j]='\0';
                len[i]=c;
                if(a[k]=='\0') break;
                i++;
                j=0;
                c=0;
            }
            else
            {
                ar[i][j]=a[k];
                c++;
                j++;
            }
        }
        for(m=0; m<=i; m++)
        {
            for(n=(len[m]-1); n>=0; n--)
            {
                printf("%c",ar[m][n]);
            }
            printf(" ");
        }
        printf("\n");
    }
    return 0;
}