#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

int maxCanAssign(vector<int>& positions, int minDistance) {
	if (positions.empty()) {
		return 0;
	}
	int result = 1;
	int previousPosition = positions[0];
	for (int positionIterator = 1; positionIterator < positions.size(); ++positionIterator) {
		int currentPosition = positions[positionIterator];
		if (currentPosition - previousPosition >= minDistance) {
			++result;
			previousPosition = currentPosition;
		}
	}
	return result;
}

void solveTest() {
	int stallsNumber, cowsNumber;
	cin >> stallsNumber >> cowsNumber;
	vector<int> stallsPositions(stallsNumber);
	for (int positionIterator = 0; positionIterator < stallsNumber; ++positionIterator) {
		cin >> stallsPositions[positionIterator];
	}
	sort(stallsPositions.begin(), stallsPositions.end());
	int minDistance = 0, maxDistance = 1000000000;
	while (maxDistance - minDistance > 1) {
		int averageDistance = (minDistance + maxDistance) / 2;
		if (maxCanAssign(stallsPositions, averageDistance) >= cowsNumber) {
			minDistance = averageDistance;
		}
		else {
			maxDistance = averageDistance;
		}
	}
	if (maxCanAssign(stallsPositions, maxDistance) >= cowsNumber) {
		cout << maxDistance << endl;
	}
	else {
		cout << minDistance << endl;
	}
}

int main()
{
	int testNumber;
	cin >> testNumber;
	for (int testIterator = 0; testIterator < testNumber; ++testIterator) {
		solveTest();
	}
	return 0;
}