Which of the following statements regarding foreign persons not engaged in a U$.$S$.$ trade or business is true?

Question $53$ options:

A$)$

They are subject to potential withholding taxes on the gross amount of U$.$S$.-$source investment income.

B$)$

If they have any U$.$S$.-$source income, they are taxed on net investment income $($after expenses$).$

C$)$

They are not subject to U$.$S$.$ tax if not engaged in a U$.$S$.$ trade or business.

D$)$

If they have only U$.$S$.-$source investment income, they are exempt from U$.$S$.$ tax.