Answer:
A.
Explanation:
because it studies the key word Astro which means astrophysics which is stars good luck 
 
        
                    
             
        
        
        
Explanation:
Un estimulo es cualquier cambio que es capaz de producir una respuesta por parte del organismo. Los receptores son estructuras muy especializadas capaces de percibir los estímulos y convertirlos en impulsos nerviosos
 
        
             
        
        
        
Secretary of defense is ahead but not a soldier in rank
In ranking its the general (which is the highest rank in the army)
        
             
        
        
        
Answer:
Tundra
Explanation:
Ocean is the most stable biome, provides the majority of earths food and oxygen. 
Rainforest provide 20% of earths food production. Also stable.
Grasslands although the don't get much rain it doesn't compare to the harsh weather that Tundra's face.
Tundra's is a very fragile environment. The freezing cold temperatures makes it a very difficult environment to survive in the winter, and plants and animals have a hard time coping with any extra stresses and disturbances. Therefore making it the least stable out of the rest listed.