The Graphs Below Refer to Two Separate Product Markets

The graphs below refer to two separate product markets.Assume that society's optimal level of output in each market is Q₀ and that government purposely shifts the market supply curve from S to S₁ in diagram (a) and from S to S₂ in diagram (b) .The shift of the supply curve from S to S₁ in diagram (a) might be caused by a per unit:

A) subsidy paid to the producers of this product.
B) tax on the producers of this product.
C) subsidy paid to the buyers of this product.
D) tax on the buyers of this product.