The correct answer is option 'B', the average revenue curve is also known as the demand curve.

The average revenue curve represents the revenue generated per unit of output sold by a firm. It shows the relationship between the average revenue and the quantity of output produced and sold by a firm. The average revenue is calculated by dividing the total revenue by the quantity of output sold.

The demand curve, on the other hand, represents the relationship between the price of a product and the quantity demanded by consumers. It shows the quantity of a product that consumers are willing and able to purchase at different prices.

The average revenue curve and the demand curve are essentially the same because the average revenue is equal to the price of the product. In a perfectly competitive market, where the firm is a price taker, the price is determined by the market forces of supply and demand. Therefore, the average revenue curve is horizontal and equal to the market price.

When the demand curve is downward sloping, the average revenue curve will also be downward sloping. This is because as the quantity of output sold increases, the price will generally decrease, leading to a lower average revenue.

On the other hand, when the demand curve is perfectly elastic, the average revenue curve will be a horizontal line. This is because the firm can sell any quantity of output at the same price, and therefore the average revenue remains constant.

In summary, the average revenue curve is also known as the demand curve because it represents the relationship between the price of a product and the quantity demanded by consumers. It shows the revenue generated per unit of output sold by a firm and is equal to the price in a perfectly competitive market.