It spread by person to person
        
                    
             
        
        
        
Answer:
Mammals
Explanation:
Mammals are a large group of animals that can be subdivided into multiple species, more than fish and insects. I cannot be D because plants are not animalia
 
        
             
        
        
        
Answer:
Biologists measure the number of trophic levels in an ecosystem so as to check its stability and health.
Explanation:
Trophic levels are the feeding position in a food chain such as primary producers, consumers, etc. They are positions that an organisms occupies in a food chain (series of steps in which organisms transfer energy by eating or being eaten);- what it eats and what eats it. 
Theoretically there is no any limit to the number of trophic levels in a food chain or number of organisms that live on each level.