Natural changed to the ecosystem relate to environmental shock that occur naturally. Natural changes include floods, volcanic activity, hurricanes, etc. They can be destructive, but they also have advantages. Floods have the contribution of nutrients to the soil which was lacking. It can also relocate fishes and organisms living in bodies of water which improves the ecosystem.
        
             
        
        
        
Learning that occurs
when an organism makes a connection between two events is called "Associative
learning".<span>
The process involves associating various details or events to
learn a particular fact or information. For example one cannot imagine one eye
of his/her father, whenever he/she will imagine the whole face of their father
will make sense to mind and not single eye.</span>
 
        
             
        
        
        
Answer: B: the variety of living things on Earth.
(Biodiversity refers to every living thing, including plants, bacteria, animals, and humans.)
        
             
        
        
        
Answer:
D. Aids
Explanation:
Aids is one of the most deadliest diseases in the world. When you contract aids, it's designed to infect the cells that try to destroy it, it's a whole zombie apocalypse in your body and you don't even know it. 
It attacks and weakens the immune system, and our immune system defends our bodies against infections, but HIV is extremely strong and it over powers the system, I'm not sure if there's an immune system strong enough to fight HIV or aids.