#include <cstdio>
#include <algorithm>
#include <vector>
#include <cstring>
#include <numeric>
#include <cassert>
#include <cmath>
#include <map>
#include <unordered_map>
#include <set>
#include <random>
#include <vector>
#include <iostream>
#include <string>
#include <complex>
#include <functional>
#pragma warning(disable:4146)

#define FOR(k,a,b) for(uint k(a); k < (b); ++k)
#define REP(k,a) for(int k=0; k < (a); ++k)
#define ABS(a) ((a)>0?(a):-(a))

using namespace std;

typedef long long LL;
typedef unsigned int uint;
typedef pair<int, int> PII;
typedef vector<int> VI;
typedef vector<double> VD;
typedef vector<VI> VVI;
typedef vector<VD> VVD;
typedef vector<vector<bool> > VVB;

int main(int argc, char** argv) {
#ifdef HOME
	freopen("in.txt", "rb", stdin);
	freopen("out.txt", "wb", stdout);
#endif
	int T,P,N;
	scanf("%d", &T);
	assert(T > 0 && T < 21);
	REP(tc, T)
	{
		scanf("%d %d", &N,&P);
		assert(N > 0 && N <= 100000);
		assert(P > 0 && P <= 100000);
		if(P < 3)
		{
			printf("impossible\n");
			continue;
		}
		string s(P, 'a');
		s[P / 2] = s[(P - 1) / 2] = 'b';
		REP(i, N / P)
			printf("%s", s.c_str());
		printf("\n");
	}
	return 0;
}