Answer:
A reaction that joins the nuclei of two atoms into one  
Explanation:
Think of the words "nuclear fusion".
Nuclear = "relating to a nucleus"
 Fusion = "joining"
So, nuclear fusion is the joining of two nuclei into one.
A and B are wrong, because chemical reactions involve electrons.
D is wrong, because the splitting of a nucleus is fission.
 
        
             
        
        
        
In a typical ECG (Electrocardiogram) the first wave obtained is P wave; The P wave represents the depolarization of the atria. After this, the QRS complex is formed. The QRS wave represents the depolarization of the ventricles.  During the depolarization phase, the ventricles contracts and pushes the blood into the atria. Hence, the correct answer is depolarization of the ventricles. 
 
        
             
        
        
        
Gorillas are ground-dwelling, predominantly herbivorous apes that inhabit the forests of central Sub-Saharan Africa. The genus Gorilla is divided into two species: the eastern gorillas and the western gorillas (both critically endangered), and either four or five subspecies. They are the largest living primates. The DNA of gorillas is highly similar to that of humans, from 95 to 99% depending on what is included, and they are the next closest living relatives to humans after the chimpanzees and bonobos.