Why are the general shapes of a firm’s average and marginal costs curves U-shaped?

because at the begining ,with the the products more and more , the cost to each product is less and less, this is quite clear ,for example ,when you have 5 workers and only make 5 product ,but when you make more ,such as 1o products, the averrage cost is less,when you make one more product,it is less too. however,if the ability of the 5 people is fixed ,when you make more and more ,they can't do it ,they will ask a higher pay, and the management is also very high ,the cost will be very high .maybe you can find a book to look up if you still can't understand.

短期平均成本曲线成U形的原因是边际收益递增和边际收益递减规律作用的原因。
长期平均成本曲线呈U形的原因是规模经济与规模不经济的原因。

边际成本曲线成U型是受到需求限制，所以规模不可能是无限扩张。

英文自己翻，我忘了名词怎么写了。