Amazon has a problem: It's too successful. It controls $44\%$ of the U$.$S$.$ e$-$commerce market, has well over $$200$ billion in annual sales, ships more than $1\mathrm{.}2$ billion packages a year, and has more than $100$ warehouses, $7,500$ truck trailers, and $40$ airplanes. However, its rapid growth has made quick and inexpensive delivery of packages crucial to its operations. In $2017,$ it spent nearly $$22$ billion on shipping globally or about $12\%$ of its overall revenue.

$1.$ To which market structure belong Amazon? Explain your answer