Manchester Corporation, a U.S. corporation, incurred $120,000 of interest expense during 2018. Manchester manufactures inventory that is sold within the United States and abroad. The total tax book value of its U.S. production assets is $15,000,000. The total tax book value of its foreign production assets is $5,000,000. What amount of interest expense is apportioned to the company's foreign source income for foreign tax credit purposes, assuming this is the first year the company makes this computation and the interest expense is fully deductible in 2018?

a. $0

b. $30,000

c. 40,000

d. $120,000