#include <bits/stdc++.h>
using namespace std;
#define int long long 
#ifdef LOCAL
void debug_out() {cerr<<endl;}
template <typename Head, typename... Tail>
void debug_out(Head _H, Tail... _T) {cerr<<" "<<to_string(_H);debug_out(_T...);}
#define debug(...) cerr<<"["<<#__VA_ARGS__<<"]:",debug_out(__VA_ARGS__)
#else
#define debug(...)
#endif
const int MAXN = 1000005;
const int inf=1000000500ll;
const long long oo =1000000000000000500ll;
const int MOD = (int)1e9+7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
typedef pair<int,int> pi; 
int n,m,C;
int A[MAXN];
int B[MAXN];
int f(int K){ // we are forced to use get exactly K stacks of vouchers for free.
	int ans=0;
	int vou=m;
	for(int i=1;i<=n;i++){
		int num=A[i]/C;
		int ai=A[i];
		if(num<=K){
			K-=num;
			vou+=num;
			ans+=num*C;
			ai-=num*C;
		} else{
			vou+=K;
			ans+=K*C;
			num-=K;
			ai-=K*C;
			K=0;
			
		}
		int vouuse=min({vou,ai});
		vou-=vouuse;
		ans+=ai-vouuse;
	}
	if(K>0)return oo;
	else return ans;
}
void solve(){
	cin>>n>>m>>C;
	for(int i=1;i<=n;i++)cin>>A[i];
	for(int i=1;i<=n;i++)cin>>B[i];

	int lower=0,upper=0;
	for(int i=1;i<=n;i++){
		upper+=A[i]/B[i];
	}
	while(upper-lower+1>=3){
		int len=(upper-lower+1);

		int c1=lower+len/3;
		int c2=upper-len/3;
		if(f(c1)<f(c2)){
			upper=c2;
		} else if(f(c1)>f(c2)){
			lower=c1;
		} else{
			lower=c1;
			upper=c2;
		}
	}
	int ans=oo;
	for(int i=lower;i<=upper;i++)ans=min(ans,f(i));
	cout<<ans<<"\n";
}
int32_t main() 
{
	ios_base::sync_with_stdio(0); cin.tie(0);
	int tc;cin>>tc;
	while(tc--)solve();
	
}