Task: Question Generation
Input (English): Context : The Buk-1 was adopted into service in 1978 following completion of state trials, while the complete Buk missile system was accepted into service in 1980 after state trials took place between 1977 and 1979. 
Subject : Buk missile system 
Relation : service entry
Output (English): Which was the official year for the approval of Buk missile system?