#include <bits/stdc++.h>
#define int long long
using namespace std;

void solve() {
	int n, k;
	cin >> n >> k;

	vector<int> a(n + 1), b(n + 1);

	int d = 0;
	vector<int> p;
	p.push_back(1);
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
		b[i] = a[i] - a[i - 1];
		if (i > 1 && d > gcd(d, b[i])) {
			p.push_back(i);
		}
		d = gcd(d, b[i]);
	}
	int ans = d;

	for (int u : p) {
		vector<int> p1;
		b[u] += k;
		int d = 0;
		for (int i = 1; i <= n; i++) {
			if (i > u && d > gcd(d, b[i])) {
				p1.push_back(i);
			}
			d = gcd(d, b[i]);
		}
		ans = max(ans, d);
		for (int v : p1) {
			b[v] -= k;
			int d = 0;
			for (int i = 1; i <= n; i++) {
				d = gcd(d, b[i]);
			}
			ans = max(ans, d);
			b[v] += k;
		}
		b[u] -= k;
	}

	cout << ans << "\n";
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int T;
    cin >> T;
    while (T--) solve();

    return 0;
}