Which of the Following Statements Regarding a Foreign Person's U

Which of the following statements regarding a foreign person's U.S. tax consequences is true?

A) Foreign persons are subject to U.S. income or withholding tax only if they are engaged in a U.S.-trade or business.
B) Foreign persons may be subject to withholding tax on U.S.-source investment income even if not engaged in a U.S. trade or business.
C) Foreign persons are not taxed on gains from U.S. real property as long as such property is not used in a U.S. trade or business.
D) Once a foreign person is engaged in a U.S. trade or business, the foreign person's worldwide income is subject to U.S. taxation.