Alone Or In A Chain

Riddle: I can come alone or in a chain. Both are special in their own way. My appearance can be unexpected or expected. What am I?

Answer: A (Chain) Reaction.

Source: https://www.riddles.com/index.php/10985

I can come alone or in a chain. Both are special in their own way. My appearance can be unexpected or expected. What am I? — Alone or in a Chain by BurntSandwich v1.

Riddle: I can come alone or in a chain. Both are special in their own way. My appearance can be unexpected or expected. What am I? Answer: A (Chain) Reaction. — by BurntSandwich v2.