Why Are Both Buyers and Sellers Price Takers in a Perfectly

Why are both buyers and sellers price takers in a perfectly competitive market?

A) Because the price is determined by government intervention and dictated to buyers and sellers.
B) Because each buyer and seller knows it is illegal to conspire to affect price.
C) Because both buyers and sellers in a perfectly competitive market are concerned for the welfare of others.
D) Because each buyer and seller is too small relative to others to independently affect the market price.