Fact 1: Fission takes place when an animal divides into two parts.

Fact 2: If the nucleus breaks up into lighter elements, it is called fission .

Given the two facts above, answer the question "What breaks up into two lighter elements when an animal divides into two parts?" with the following options: 
- the blood cells
 - the heart
 - Energy.
 - the main element
 - heat energy
 - light energy
 - the nucleus
 - sensory neurons


 

       the nucleus  