Friction is a force that works against motion. While walking, friction helps us maintain our stability and prevents us from slipping. Friction also works the same way with cars and vehicles in general, preventing them from skidding and helping them stop when the brakes are applied. We also need friction to make a fire.
 
        
                    
             
        
        
        
The best and most correct answer among the choices provided by your question is the first choice or letter A. <span>A biological agent that cannot reproduce on its own is a VIRUS.
</span>A virus<span> is a small infectious agent that replicates only inside the living cells of other organisms.
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Answer:
3. Rocks erode and sediment is deposited.
4. No sediment is deposited over a long time.
Explanation:
An unconfomity is a break in stratigraphic record of a place usually as a result of non-deposition or erosion. 
In a sequence of sedimentary rock strata some surfaces called unconformity surfaces gives clues about the depositional history of a particular place. During an unconfomity, there is usually no sediment deposition and when deposition resumes, the underlying rocks and the new ones will not share the same attribute especially by age. Unconformity can also be as a result of erosion of a sequence. 
In the case of non- conformity, a sedimentary rock is deposited above an igneous or sedimentary rock. Option 5 is a simple example of an igneous intrusion called a sill. It is not unconformity. 
 
        
                    
             
        
        
        
C. Tree roots split apart rocks in the ground