Within the supply-and-demand model, a tax collected from the buyers of a good shifts the a. supply curve upward by the size of the tax per unit b. demand curve downward by the size of the tax per unit c. supply curve downward by the size of the tax per unit d. demand curve upward by the size of the tax per unit

In the supply-and-demand model, when a tax is collected from the buyers of a good, it effectively increases the price that buyers have to pay for the good. This change in price affects the demand for the good. Here's the reasoning for each option: a. **Supply curve upward by the size of the tax per unit**: This is incorrect because the tax is imposed on buyers, not suppliers. b. **Demand curve downward by the size of the tax per unit**: This is correct. The tax increases the effective price that buyers pay, which reduces the quantity demanded at any given price, shifting the demand curve downward. c. **Supply curve downward by the size of the tax per unit**: This is incorrect because the tax does not directly affect the supply side in this case. d. **Demand curve upward by the size of the tax per unit**: This is incorrect because the tax does not increase demand; it decreases it. Therefore, the correct answer is: **b. demand curve downward by the size of the tax per unit**.