Patents, limiting the number of licenses available, and economies of scale can all hinder a firm's ability to enter the market. A patent prevents a firm from replicating a product that originated from another firm. A limited number of licenses can exclude firms who are unable to obtain licenses from entering the market. Economies of scale can prevent smaller firms from entering the market by making such an action cost-prohibitive. 

All of these are examples of factors of production, which are the inputs necessary for a production process. 

When both goods are perfect substitutes, the consumer is indifferent between the amount of consumption of the two goods. Thus, his/her indifference curves will be straight lines, because the rate of substitution between the two goods will be constant. 

In perfectly competitive markets, there are low barriers to entry (think of a food truck--all one needs in order to operate are a truck, fuel, cooking equipment, and a business license), homoegenous products (consider a farm--nearly all cucumbers are identical to one another), and a large number of firms (this is what makes the environment so competitive).

In monopolistic competition and oligopolies, products tend to be heterogeneous, and in a monopoly, there are extremely high barriers to entry.

The marginal revenue curve always has the same intercept on the price axis as the demand curve and twice the slope of the demand curve.

For market structures of oligopoly, monopoly, and monopolistic competition, the firm faces a demand curve that is downward sloping (indicated market power), so the marginal revenue curve would be steeper than the demand curve.

For perfectly competitive market structures, however, the firm faces a demand curve that is horizontal, with a slope of 0 (i.e. it is perfectly elastic). For a firm in perfect competitive market structure, therefore, the marginal revenue curve, which has the same intercept and twice the slope, results in the exact same curve as the demand curve.

A firm will hire labor until the marginal revenue product of labor is lower than the cost of labor/wage, which is $8. The marginal revenue product of labor is the marginal product of labor multiplied by the product's price. The marginal revenue of the fourth unit of labor is $10 (five units multiplied by $2) and the marginal revenue of the fifth unit of labor is $6 (three units multiplied by $2). Thus, the firm will hire four units of labor.

A profit maximizing firm will hire labor until the marginal product of labor is greater than the wage rate. If the marginal product of labor is greater than the wage rate, then the firm should hire more labor until the two values are equal.

Like any market, the profit maximization level is the point at which marginal cost equals marginal revenue. In this case, the marginal cost of labor is the labor wage. Thus, the profit maximization point for the employer is the point at which wage equals the marginal revenue product of labor.

The marginal revenue product of labor for the fourth unit of labor is its marginal product multiplied by the cost of the product. The marginal product of the fourth unit of labor is 4 (the difference between total production at four units of labor and three units of labor), and cost of the product is $2, so the marginal revenue product of labor for the fourth unit is $8.

The firm will hire three units of labor, because it will continue to hire until the wage exceeds the marginal revenue product of labor (MRPL). The MRPL after the third unit of labor is less than $10, which means that the firm will not hire more than three units of labor.