In each of the following cases, determine if the United States has jurisdiction to tax Mr. KT.
a. Mr. KT is a U.S. citizen but has been a permanent resident of Belgium since 1993.
b. Mr. KT is a citizen and resident of Canada. He owns an apartment building in Buffalo, New York, that generates $18,000 annual net rental income.
c. Mr. KT is a citizen of Singapore but is a permanent resident of St. Louis, Missouri.
d. Mr. KT, a citizen and resident of Greece, is a partner in Sophic Partnership, which conducts business in 12 countries, including the United States.