You are a managing a network of compiters. Every computer in this network needs to access a core database, which contains data necessary for basic operatinh system function. You can store a copy of the database in each computer, but this requires you to maintain all these copies. You can store just one copy in one specific computer, but then orher computers need to request accesses through the network, which may cause considerable network congestion $1.$ You can store a copy of the database oon each of the selected computers $2.$ Each computer that is not selected has a direct link to at least one of the selected computers. Prove that L is NP$-$hard