If we use Radix Sort to sort $n$ integers in the range $\left (n^{k/2}, n^k \right ]$, for some $k > 0$ which is independent of $n$, the time taken would be?
1. $\Theta(n)$
2. $\Theta(kn)$
3. $\Theta(n \log n)$
4. $\Theta(n^2)$