When comparing lithium, sodium, and potassium to calcium, strontium, and barium, early chemists noticed that:

Explanation:

Lithium, Potassium and Sodium follow each other in that order when it comes to reactivity.

Succinctly put, Lithium is the most reactive. Potassium is more reactive and Sodium is the least reactive.

Also, for Calcium, Strontium and Barium. It is the least reactive in this order.

Strontium, Barium, Calcium

That is, Strontium is more reactive than Barium, which is more reactive than Calcium. Calcium is the least reactive of the 3

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.