Garg (FOCS '96) gives two approximation algorithms for the 
minimum-cost tree spanning $k$ vertices in an undirected graph.  
Recently Jain and Vazirani (JACM '01) discovered primal-dual 
approximation algorithms for the metric uncapacitated facility 
location and $k$-median problems.  In this paper we show how 
Garg's algorithms can be explained simply with ideas introduced 
by Jain and Vazirani, in particular via a Lagrangean relaxation 
technique together with the primal-dual method for approximation 
algorithms.  We also derive a constant factor approximation 
algorithm for the $k$-Steiner tree problem using these ideas,
and point out the common features of these problems that allow
them to be solved with similar techniques.