The definition of a trial is  a hearing in a court case, a test or a hardship. An example of trial is the court case of an accused robber. An example of trial is the use of a small size bottle of shampoo to sample the product.
An example of a market that meets all four conditions for perfect competition is: wheat. A market structure with many sellers and many buyers is: perfect competition. Offering products of different tastes and shapes is an example of: nonprice competition. An example of a barrier to entry is: high start-up costs. Compared to a market with perfect competition, a monopoly often has: higher prices and fewer goods. In many industries, deregulation has resulted in: lower prices for consumers. A market that is an oligopoly has: a few firms dominating the market.
For example, if many people can enjoy the same good at the same time (non-rival, non-excludable consumption), then private markets may supply too little of that good. National defense is one example of non-rival consumption, or of a public good. Market failure occurs when private markets do not allocate goods or services efficiently. The existence of market failure provides an efficiency-based rationale for collective or governmental provision of goods and services.