6. NASA wants to link N stations spread over the country using communication chan nels. For every pair of stations, there is a badnwidth associated with the channel between them, which is known a priori. It (NASA) wants to select N - 1 channels (the minimum possible) in such a way that all the stations are linked by the channels and the total bandwidth (defined as the sum of the individual bandwidths of the channels) is maximum. Give an efficient algorithm for this problem and determine its worst-case time complexity. (20 pts.)