Which of the following statements regarding the U.S. taxation of non-U.S. persons is true?

a. A non-U.S. person may earn income from selling U.S. real property without incurring any U.S. income tax

b. A non-U.S. person's effectively connected U.S. business income is taxed by the United States only if it is portfolio income

c. A non-U.S. person must spend at least 183 days in the United States before any effectively connected income is subject to U.s taxation.

d. A non-US. person's effectively connected U.S. business income is subject to U.S. income taxation.