When Ruby practice piano she gets a sticker, and when she does a chore she gets 2 stickers. When she has 20 stickers her parents will buy her a
pizza as reward. This system is called?

A reward system.

Explanation:

My parents use this system all the time and they say that it is called a reward system.

industry had made the u.s. the richest nation in the world at this time. the second industrial revolution caused growth in industry and transportation, which allowed increased trade between nations.