Mutation are two types somatic and germ cell mutation...as there is mutation in germ cell it will alter the normal behavior and produce variation !
random fertilization also causes variation as different setup of genes meets with another different...so chances of variation increases !
and third crossing over is in meiosis when chiasmata forms....so in this two homologous chromosome attach to each other and on separation ... some part exchange so leads to variation !
        
             
        
        
        
As humans sometimes experience, a cold environment can result in<span> </span>shivering<span>. This is when skeletal muscles begin to shake in small movements, creating warmth by expending energy. Humans can also sweat in high temperatures. The water on the surface of the skin is able to absorb a lot of heat during evaporation, resulting in cooling of the body.</span>
        
             
        
        
        
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). 
 
        
             
        
        
        
Atom, cell, tissue, organ
        
             
        
        
        
Answer:
false
Explanation:
im not sure if this is the entire question but from what I can see, I would prove this false