Final answer:
The statement that is not true in the range of the total revenue curve labeled A is that price is constant, because in a perfectly competitive market, marginal revenue equals price and is constant, while total revenue increases with each additional unit sold.
Step-by-step explanation:
The answer to the student's question about which statement is not true in the range of the total revenue curve labeled A is that price is constant. In a perfectly competitive market, the demand curve faced by a firm is perfectly elastic, meaning that the firm can sell as many units of the product as it wishes at the market price. In such a market, marginal revenue is equal to price, and any increase in the quantity sold results in a proportional increase in the total revenue, keeping the price constant.
When the total revenue is increasing and the elasticity is unitary, it means that the percentage change in quantity demanded is equal to the percentage change in price, which is a characteristic of constant unitary elasticity. In this case, the curve should be a straight line moving upwards from the origin. However, if marginal revenue were zero, this would imply that selling additional units does not increase total revenue, which differs from the scenario in a perfectly competitive market where the additional revenue from selling one more unit is exactly equal to the price of that unit.