Valves, they are like flaps of muscle that keeps the blood in its place util its ready to be pumped along.
        
                    
             
        
        
        
Lipids are found in cell membranes in double layer which is called "Lipid-bilayer".
So, option A is your answer.
Hope this helps!
        
             
        
        
        
Answer: Option (A) is the correct answer.
Explanation:
Beaches are the places which are part of shore of an ocean, sea, large river etc.
There are pebbles and sand at the shore of a beach. Waves of water in an ocean always change the ocean due to  erosion and addition of sand.
It is the force acted by the wave that pushes or pull the sand in different directions every time the wave comes.
So we can conclude that the statement beaches are always changes due to factors such as waves adding and eroding sand best describes beaches.
 
        
                    
             
        
        
        
Answer:
Wakefulness and sleep.
Explanation:
Hypothalamus may be defined as the small region of the brain that maintains the homeostasis of the body. The hypothalamus is divided into anterior hypothalamus and posterior hypothalamus.
The anterior hypothalamus of the brain is involved in the sleep activity of the organisms and releases the hormones also. The posterior hypothalamus is involved in the wakefulness of the individual.
Thus, the correct answer is wakefulness and sleep.