Answer:
d. nitrogen-containing base
.
Explanation:
A nucleotide comprises a phosphate group, a pentose sugar and a nitrogenous base (nitrogen-containing base).
A nitrogenous base is a type of aromatic heterocyclic organic compound which essentially has nitrogen in it. The base can be a purine or pyrimidine. Purine nitrogenous bases are adenine (A) and guanine (G) while pyrimidine nitrogenous bases are cytosine (C), thiamine (T) and uracil (U). 
Till the time a base is not attached to covalently linked phosphate group and a pentose sugar, it is known as nucleoside. But as soon as we attach any nitrogenous base to the nucleoside, a nucleotide is formed. So it means a nucleotide is nucleoside + nitrogenous base.
Examples are as under:
A nucleotide which has adenine as nitrogenous base is known as adenosine triphosphate (ATP) while a nucleotide which has guanine as nitrogenous base is known as Guanosine-5'-triphosphate (GTP). 
 
        
             
        
        
        
Answer:
B. an asteroid belt lies between them.
Explanation:
The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets.
 
        
             
        
        
        
Two factors infuence the regions photosynthetic productivity which would be the amount of solar radiation and the availability of nutrients because solar radiation is needed as well as nutrients which keeps the plant healthy 
hope this helps (:
        
             
        
        
        
Answer: 
    Oral poliovirus vaccine or Sabin vaccine, named for its inventor American physician and microbiologist Albert Sabin, was developed. OPV contains live attenuated (weakened) virus and is given orally.