5. A reaction in which them material that starts the reaction is also one of the products and can start another reaction is called a. An accelerator reaction
b. A moderator reaction
c. A shielding reaction
d. A chain reaction

chain reaction

Explanation:

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.