//skp
#include <bits/stdc++.h>
#define ll long long
#define fastio ios_base::sync_with_stdio(false), cin.tie(nullptr)
using namespace std;
const ll mod = 1000000007;

int main()
{
    fastio;
    ll tc = 1;
    cin >> tc;
    for (ll t = 0; t < tc; t++)
    {
        ll n, k;
        cin >> n >> k;
        string a, b;
        cin >> a;
        cin >> b;
        sort(a.begin(), a.end());
        sort(b.begin(), b.end());
        vector<char> aa, bb;
        bool flag = 0;
        for (ll i = 0; i < n; i++)
        {
            if (a[i] != b[i])
            {
                aa.push_back(a[i]);
                bb.push_back(b[i]);
                if(a[i]>b[i])
                    flag = 1;
            }
        }

        map<char, ll> m, m1;
        for (ll i = 0; i < aa.size(); i++)
        {
            m[aa[i]]++;
            m1[bb[i]]++;
            if (aa[i] == 'z')
            {
                flag = 1;
            }
        }
        if (flag)
            cout << "No\n";
        else
        {
            ll freq = INT_MAX, freq2 = INT_MAX;
            for (ll i = 0; i < aa.size(); i++)
            {
                freq = min(freq, m[aa[i]]);
                freq2 = min(freq2, m1[bb[i]]);

            }
            if (freq < k || freq2<k)
                cout << "No\n";
            else
                cout << "Yes\n";
        }
    }
    return 0;
}