The cell membrane or cell wall 
        
                    
             
        
        
        
Answer:
 A) actin and myosin  is a correct answer.
Explanation:
- Myofibrils are the contractile fiber and they are made of actin and myosin.
-  Myofibrils are made of thick myofilaments and thin myofilaments.
-  Thick myofilaments are made up of myosin whereas thin     myofilaments are composed of actin.
- The functional unit of muscle is Myofibrils and contraction of muscle occurs due to myofibril, when the filaments actin and myosin interact with each other.
Thus Myofibrils are made primarily of (A) actin and myosin.
 
        
             
        
        
        
These are all caused by viruses.
        
             
        
        
        
It is the answer c hope this help