Answer: Option A "Choroid plexus".
Explanation: 
Cerebrospinal fluid can be defined as the fluid that is found in the brain and spinal cord of human beings. It is a clear and colorless fluid.
The production of cerebrospinal fluid takes place by the specialized ependymal cells that lies in the choroid plexuses which is present in the  ventricles of brain.
The amount of cerebrospinal fluid in a day is 500mL. This fluid acts as cushion that protects the brain from mechanical and immunological protection.
Hence, the correction answer is option A 
 
        
             
        
        
        
Answer:
i
Explanation:
long chains of repeating subunits, called nucleotides
A nucleotide is the basic building block of nucleic acids
 
        
             
        
        
        
Answer:  phylum
Explanation:
kingdom, phylum, class, order, domain, family, genus and species
 
        
                    
             
        
        
        
Option A I guesss. Not sure