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

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

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

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

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