In dobereiner's classification system, the atomic mass of the middle element could be estimated by a. finding the difference between the atomic masses of the other two elements in a triad b. finding the product of the atomic masses of the other two elements in a triad c. averaging the atomic masses of the heaviest and lightest elements in a triad d. adding the atomic masses of the other two elements in a triad

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.