Before revision:  It is not hard to see the clustered selected-internal Steiner tree problem is NP-hard. Hence, in this paper, we present the first known approximation algorithm with performance ratio  \rho+4  for the clustered selected-internal Steiner tree problem if the cost function is metric (i.e., the costs of edges satisfy the triangle inequality), where \rho is the best-known performance ratio for the Steiner tree problem whose performance ratio is \ln 4 +\epsilon\approx 1.39 .
After revision:  In this paper, we present the first known approximation algorithm with performance ratio \rho+4 for the clustered selected-internal Steiner tree problem if the cost function is metric (i.e., the costs of edges satisfy the triangle inequality), where \rho is the best-known performance ratio for the Steiner tree problem whose performance ratio is \ln 4 +\epsilon\approx 1.39 .