4. Perfect (1^st degree) price discrimination is essentially impossible, though some types of sales behavior attempt to achieve this. Since under 1^st degree price discrimination each unit is sold at a different price, and at the highest price the consumer is willing to pay, the firm will keep selling until the price (which is also MR) of the last unit equals its MC. For this firm the last unit sold will be the 7^th unit at a price of 8, since that is the last unit for which P is not below MC.