The Figure Shows the Market for Shirts in the United

The figure shows the market for shirts in the United States, where D is the domestic demand curve and S is the domestic supply curve. The world price is $20 per shirt. The United States imposes a tariff on imported shirts of $4 per shirt.
-In the figure above, the U.S. government's revenue from the tariff is

A) $64 million.
B) $32 million.
C) $128 million.
D) $48 million.