Final answer:
The Upper Assembly and Lower Assembly support, protect, and house various assemblies of the GAU-21 Machine Gun.
Step-by-step explanation:
The correct answer is b) Upper; Lower.
The Upper Assembly and Lower Assembly support, protect, and house various assemblies of the GAU-21 Machine Gun. The Upper Assembly includes components like the barrel, receiver, and feeding system, while the Lower Assembly includes components like the grip, trigger group, and stock.
For example, the Upper Assembly provides the necessary structure for the barrel to be attached and aligned properly, while the Lower Assembly houses the trigger group, allowing the gun to be fired.