partial

Absence of transport cost

oligopoly

Both (a) and (b)

Marginal Revenue

How price (AR) lesser than MC is ruled out for a perfectly competive firm in a state of equilibrium. Equality between AR and MC is struck at point Q where all the conditions of equilibrum are satisfied. Suppose the firm decides to produce OL₁ output where AR(=L₁T) < MC (= L₁Q₁). As a cpmsequence of shifting from Q to Q₁, incremental revenue (=LL₁TQ) < incremental cost (=LL₁Q₁Q). Incrementa; cost is greater than incremental revenue by the area QTQ₁. Implying that the differential between TR and TC will reduce in case the firm shifts from Q to Q₁. Or profit is maximised only at Q whereprice = MC, not at any point where AR > MC 1.

A perfectly competitive market is dominated by the presence of a large number of buyers and sellers of a commodity, which means that there is no such buyer or seller in the market whose purchase or sale is so large as to impact the total sale or purchase in the market.Each buyer/seller has only a fractional share in the market demand/market supply.Hence, price is determined by the forces of market demand and market supply.No individual buyer or seller has any control over it.Each buyer/seller has to accept the price as it is in the market. 
Hence, price is determined by the forces of market demand and market supply.No individual buyer or seller has any control over it.Each buyer/seller has to accept the price as it is in the market. 

A perfectly competitive market is dominated by the presence of a large number of buyers and sellers of a commodity, which means that there is no such buyer or seller in the market whose purchase or sale is so large as to impact the total sale or purchase in the market.Each buyer/seller has only a fractional share in the market demand/market supply.
Hence, price is determined by the forces of market demand and market supply. No individual buyer or seller has any control over it.Each buyer/seller has to accept the price as it is in the market. 

(i) Product differentiation It is a distinct feature of monopolistic competition.A product is often differentiated by way of trademarks and brand names.The differentiated products are close substitutes of each other like colgate and closeup toothpaste.

Because of product differentiation, each firm can influence its price.So that, each firm has a partial control over price of its product.

(ii) Free entry or exit of firms Firms are free to enter the industry or leave it.However, new firms have no absolute freedom of entry into industry.Products of some firms may be legally patented.New firms cannot produce those products, e.g. no rival firm can produce or sell a patented item like woodland shoes.

In the Perfectly Competitive market situation.

It is MC curve of the firm starting from a point where MC = AVC (minimum). In Figure, short period supply curve of the firm is MC curve starting from point Q where AR = AVC (minimum).

Elasticity of Demand measures the degree of responsiveness in quantity demanded due to change in own price of the product(price elasticity) or change in the income of the consumer(income elasticity)or change in the price of related goods(cross elasticity).There are many factors that affect Elasticity of Demand and market form is definitely an important factor affecting Elasticity of Demand.

Elasticity of Demand in different market forms is given below:

Related goods are of two types:

(i) Substitute goods These goods can be used one in place of another, like tea and coffee.

(ii) Complementary goods These goods have to be used together in a fixed proportion like car and petrol.

So, from the above table, we can interpret that substitute goods exhibit a positive cross price elasticity for demand and negative cross price elasticity of supply.On the other hand, complementary goods exhibit a negative cross price elasticity for demand and positive cross price elasticity of supply.