Answer:
The correct answer is A    DNA  is replicated before mitosis begins
Explanation:
DNA replication is one of the most significant event of a cell cycle. .DNA replication occur in the S phase or synthesis phase of the cell cycle.
        DNA is replicated once per cycle to maintain to normal chromosome number in daughter cells.
      DNA is replicated to form two new DNA molecules which are then equally distributed in each daughter cell during the mitosis phase.
 
        
             
        
        
        
Answer:
a dam flooding a valley
Explanation:
It would be a dam flooding a valley because in a new ecosystem there would then be a increased load of phosphorus and nitrogen and increased algae growth. 
 
        
             
        
        
        
Answer:
Diffusion 
Explanation:
Diffusion is the spontaneous movement of gases, without the use of any energy or effort by the body, between the alveoli and the capillaries in the lungs.