Which of the following statements regarding the sourcing of gross income is true?

a. All income earned by non-U.S. persons not engaged in a U.S. trade or business is treated as foreign source.

b. U.S.-source income is not subject to withholding so long as such income is not treated as effectively connected with a U.S. trade or business.

c. Certain U.S.-source investment income earned by non-U.S. persons not engaged in a U.S. trade or business may be subject to a U.S. withholding tax.

d. Non-U.S. persons not engaged in a U.S. trade or business are indifferent as to whether any of their income is U.S. source.