Which of the Following Statements Regarding a Non-U

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

A) Non-U.S. persons may be subject to withholding tax on U.S.-source investment income even if not engaged in a U.S. trade or business.
B) Non-U.S. persons are subject to U.S. income or withholding tax only if they are engaged in a U.S. trade or business.
C) Non-U.S. 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 non-U.S. person is engaged in a U.S. trade or business, the non-U.S. person's worldwide income is subject to U.S. taxation.