Klassen Toy​ Company, Inc., assembles two parts​ (parts 1 and​ 2): Part 1 is first processed at workstation A for 10 minutes per unit and then processed at workstation B for 20 minutes per unit. Part 2 is simultaneously processed at workstation C for 30 minutes per unit. Work stations B and C feed the parts to an assembler at workstation​ D, where the two parts are assembled. The time at workstation D is 15 minutes. ​a) The bottleneck of this process is workstation D ​, at 4 minutes per unit ​(enter your response as a whole​ number).