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

#define fast ios_base::sync_with_stdio(false); cin.tie(NULL);
#define mp make_pair
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define all(x) (x).begin(), (x).end()
#define fr(i,a,n) for(lli i=a;i<(n);i++)
#define frj(j,a,n) for(lli j=a;j<(n);j++)
#define frx(i,a,n) for(lli i=a;i>=(n);i--)
#define frxj(j,a,n) for(lli j=a;j>=(n);j--)
#define lli long long int
#define ld long double
#define vecI vector<lli>
#define vecP vector<pair<lli,lli>>
#define Pair pair<lli,lli>
#define out2(x, a) << x << a
#define out1(x) << x
#define br << endl 
#define itr iterator


void solve() {
    lli n;
    string s;
    cin >> n >> s;
    vecI v(26);
    fr(i,0,n) {
        v[s[i]-'a']++;
    }
    
    fr(i,0,26) {
        fr(j,0,v[i]) {
            cout << (char) (i+'a') ;
        }
    } cout br;
}


int main() {
    lli t;
    cin >> t;
    while(t--) {
        solve();
    }
}