When McDonald's introduced its $1 $2 $3 menu in 2020 and competitors responded by putting popular items on their value meal menus to keep up, this was an example of a(n) that results when several competitors use promotions simultaneously.

Google started as one of many internet search engines, amazon started as an online book seller, and ebay began as a site where people could sell used personal items in auctions. these firms have grown to be so large and dominant that they are facing antitrust scrutiny from competition regulators in the us and elsewhere. did these online giants grow by fairly beating competition, or did they use unfair advantages? are there any clouds on the horizon for these firms -- could they face diseconomies of scale or diseconomies of scope as they continue to grow? if so, what factors may limit their continued growth?