#include<algorithm>

#include<cstring>

#include<cstdio>

#include<iostream>

using namespace std;

const int N=1e5+;

int n,T;

long double ans;

double m;

double a[N];

int main()

{

    freopen("atom.in","r",stdin);

    freopen("atom2.out","w",stdout);

    scanf("%d",&T);

    while (T--)

    {

        scanf("%d%lf",&n,&m);ans=-;

        for (int i=;i<=n;i++) scanf("%lf",a+i);

        sort(a+,a++n);

        int id=;

        for (int i=;i<n-;i++)

        {

            for (id=max(id,i+);id<n&&a[id+]-a[i]<=m;id++);

            if (a[id]-a[i]<=m) ans=max(ans,(long double)(a[id]-a[i+])/(a[id]-a[i]));

        }

        if (ans==-) puts("-1");

        else printf("%.13lf\n",(double)ans);

    }

    return ;

}