Suppose that fabulous had purchased trucks for $400,000 which were used solely in the transportation of inventory purchased in different parts of the united states to U.S. customers. all transportation expenses, including depreciation deductions of $200,000 in respect of the trucks, were applied to reduce U.S.-source income. Fabulous then resells the used trucks for $410,000, thereby realizing income of $210,000, to a foreign purchaser that took title to the trucks in Peru. What is the source of the income realized on the sale of the trucks?