x __ A demand curve is a willingness -and-ability-to-pay curve tells us that marginal benefit rises as the quantity purchased increases is a marginal cost curve shows us that the highest price that someone is willing to pay for a an additional unit of a good or service rises as the quantity increases

## Step 1<br />The problem is asking us to identify the correct statement about a demand curve. A demand curve is a graphical representation of the relationship between the price of a good or service and the quantity of that good or service that consumers are willing to purchase at that price.<br /><br />## Step 2<br />The first option states that a demand curve is a willingness-and-ability-to-pay curve. This is not entirely correct. While a demand curve does reflect the willingness of consumers to pay for a good or service, it does not necessarily reflect their ability to pay.<br /><br />## Step 3<br />The second option suggests that a demand curve tells us that marginal benefit rises as the quantity purchased increases. This is also incorrect. In fact, the opposite is true. As the quantity purchased increases, the marginal benefit typically decreases.<br /><br />## Step 4<br />The third option suggests that a demand curve is a marginal cost curve. This is not correct. A marginal cost curve represents the cost of producing one more unit of a good or service, not the demand for that good or service.<br /><br />## Step 5<br />The fourth option states that a demand curve shows us that the highest price that someone is willing to pay for an additional unit of a good or service rises as the quantity increases. This is correct. As the quantity of a good or service increases, the maximum price that a consumer is willing to pay for an additional unit typically decreases. This is known as the law of diminishing marginal utility.