/*

POJ 3080 Blue Jeans

KMP

*/

#include <iostream>

#include <vector>

#include <string>

using namespace std;

#define REP(i,n) for(int i=0;i<(n);++i)

#define FOR(i,a,b) for(int i=(a);i<=(b);++i)

#define RFOR(i,a,b) for(int i=(a);i>=(b);--i)

vector<int> pi;

void build(string pattern) {

int lp = pattern.length();

pi.assign(lp, -1);

vector<int> pi(lp, -1);

int k = -1;

FOR(q,1,lp-1) {

while (k >= 0 && pattern[k + 1] != pattern[q]) k = pi[k];

if (pattern[k + 1] == pattern[q]) ++k;

pi[q] = k;

}

}

bool StringMatchKMP(string text, string pattern) {

int lt = text.length(), lp = pattern.length();

int k = -1;

REP(i,lt) {

while (k >= 0 && pattern[k + 1] != text[i]) k = pi[k];

if (pattern[k + 1] == text[i]) ++k;

if (k == lp - 1) return true;

}

return false;

}

void run() {

int n;

cin >> n;

vector<string> mm(n);

REP(i,n) cin >> mm[i];

RFOR(len,60,3) {

string seq = "";

REP(i,60-len+1) {

string pat = mm[0].substr(i,len);

build(pat);

bool match = true;

FOR(j,1,n-1) {

if (!StringMatchKMP(mm[j], pat)) {

match = false;

break;

}

}

if (match) {

if (seq == "" || pat < seq) seq = pat;

}

}

if (seq != "") {

cout << seq << endl;

return;

}

}

cout << "no significant commonalities" << endl;

}

int main() {

int kase;

cin >> kase;

while (kase--) {

run();

}

return 0;

}