#include "bits/stdc++.h"
using namespace std;

#define ll long long 
#define INF 0x3f3f3f3f
#define LNF 0x3f3f3f3f3f3f3f3fll
#define MOD 998244353
#define pii pair<int, int>

#define N 100010
#define K 20

vector<int> factors(int x) {
    vector<int> res;
    int s = sqrt(x);
    for (int i = 1; i <= s; ++ i) {
        if (x % i == 0) {
            res.emplace_back(i);
            if (x/i != i) res.emplace_back(x/i);
        }
    }
    return res;
}

ll gcd(ll a, ll b) {
    return b == 0 ? a : gcd(b, a % b);
}

int main() {
#ifdef TEST
    freopen("zz.in", "r+", stdin);
#endif
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    int t; cin >> t;
    while (t --) {
        ll n, k; cin >> n >> k;
        vector<ll> a(n+5);
        for (int i = 1; i <= n; ++ i) cin >> a[i];

        auto ok = [&](ll x) {
            int lb = -1, rb = -1, cnt = 0;
            for (int i = 1, j = 1; i <= n; ++ i) {
                if (a[i] % x == 0) continue;
                while (j <= i || (j <= n && (a[j] % x != 0))) ++ j;
                lb = i, rb = j, ++ cnt;
                i = j - 1;
            }

            int res = cnt == 1;
            for (int i = lb; i < rb; ++ i) res &= (a[i] + k) % x == 0;
            return res;
        };

        ll res = a[1] + k;
        for (int i = 1; i <= n; ++ i) res = gcd(res, a[i] + k);

        unordered_set<int> s;
        for (int i = 1; i <= n; ++ i) {
            for (auto x : factors(a[i])) {
                s.insert(x);
            }
        }
        
        assert(s.size() <= 1e4);

        for (auto x : s) if (ok(x)) res = max(res, 1ll * x);
        cout << res << '\n';
    }
}