Before revision:  We present a polynomial-time algorithm with an approximation factor of O(k \log \frac{w_{max}{w_{\min}}} ) to the optimal solution, where w_{ max } and w_{ min } are the maximum and minimum weight of the sites respectively.
After revision:  We present a polynomial-time algorithm with an approximation factor of O(k ^2 \log \frac{w_{\max}{w_{\min}}} ) to the optimal solution, where w_{ max } and w_{ min } are the maximum and minimum weight of the sites respectively.