Within the supply and demand model, a tax collected from the buyers of a good shifts the ?

Within the supply and demand model, a tax collected from the buyers of a good shifts the ?

A. supply curve downward by the size of the tax per unit.
B. Supply curve upward by the size of the tax per unit
C. demand curve upward by the size of the tax per unit.
D. demand curve downward by the size of the tax per unit