//Author: Kevin
#include<bits/stdc++.h>
//#pragma GCC optimize("O2")
using namespace std;
#define ll long long
#define ull unsigned ll
#define pb emplace_back
#define mp make_pair
#define ALL(x) (x).begin(),(x).end()
#define rALL(x) (x).rbegin(),(x).rend()
#define srt(x) sort(ALL(x))
#define rev(x) reverse(ALL(x))
#define rsrt(x) sort(rALL(x))
#define sz(x) (int)(x.size())
#define inf 0x3f3f3f3f
#define pii pair<int,int>
#define lb(v,x) (int)(lower_bound(ALL(v),x)-v.begin())
#define ub(v,x) (int)(upper_bound(ALL(v),x)-v.begin())
#define uni(v) v.resize(unique(ALL(v))-v.begin())
#define longer __int128_t
void die(string S){puts(S.c_str());exit(0);}
int u[303],v[303];
int n,m;
int c[303];
mt19937 rnd(114514);
vector<pii> G[105];
bool vis[105];
void dfs(int u,int d=0)
{
vis[u]=1;
for(auto pr:G[u])
if(!vis[pr.first])
{
c[pr.second]=d;
dfs(pr.first,u,d^1);
}
}
void color()
{
memset(vis,0,sizeof(vis));
memset(c,-1,sizeof(c));
dfs(rnd()%n+1,0);
for(int i=1;i<=m;i++)
if(c[i]==-1)
c[i]=rnd()%2;
}
bitset<256> E[256];
bool check()
{
for(int i=1;i<=n+n;i++)
E[i]=0;
for(int i=1;i<=m;i++)
E[c[i]*n+u[i]][(c[i]^1)*n+v[i]]=E[c[i]*n+v[i]][(c[i]^1)*n+u[i]]=1;
for(int i=1;i<=n;i++)
{
bitset<256> cur;
queue<int> q;
q.push(i);
q.push(n+i);
cur[i]=cur[n+i]=1;
while(!q.empty())
{
int x=q.front();
q.pop();
bitset<256> bs=E[x]^(E[x]&cur);
cur|=bs;
int p=bs._Find_first();
while(p<=n+n)
{
q.push(p);
p=bs._Find_next(p);
}
}
for(int j=1;j<=n;j++)
if(!cur[j]&&!cur[j+n])
return false;
}
return true;
}
void solve()
{
for(int i=1;i<=n;i++)
G[i].clear();
for(int i=1;i<=m;i++)
{
G[u[i]].pb(v[i],i);
G[v[i]].pb(u[i],i);
}
int threshold=30000;
while(threshold--)
{
color();
if(check())
{
for(int i=1;i<=m;i++)
putchar(c[i]?'R':'B');
putchar(10);
return ;
}
}
puts("IMPOSSIBLE");
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int t;
cin>>t;
while(t--)
{
cin>>n>>m;
for(int i=1;i<=m;i++)
cin>>u[i]>>v[i];
solve();
}
return 0;
}