#include <iostream>
#include <string.h>
#include<map>
#include<set>
#include<vector>
#include<queue>
#include<math.h>
#include<algorithm>
using namespace std;
struct video {
int time;
int value;
}a[1000];
//bool cmp(const video&a,const video&b)
//{
// return a.time < b.time;
//}
int q;
int n, t;
int main()
{
cin >> q;
while (q--)
{
cin >> n >> t;
/* memset(a, sizeof(a), 0);*/
int ans = 0;
int k = 0;
int key = 0;
for (int i = 1; i <= n; i++)
{
cin >> a[i].time;
}
for (int i = 1; i <= n; i++)
{
cin >> a[i].value;
}
for (int i = 1; i <= n; i++)
{
if (a[i].value > ans && a[i].time <= t)
{
ans = a[i].value;
k = i;
key = 1;
}
t--;
}
if (key == 0)
{
cout << -1 << endl;
}
else
{
cout << k << endl;
}
}
return 0;
}
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