In each of the following cases, determine if the United States has jurisdiction to tax Mr. Tompkin. 

a. Mr. Tompkin is a U.S. citizen but has been a permanent resident of Belgium since 1993. 

b. Mr. Tompkin 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. Tompkin is a citizen of Singapore but is a permanent resident of St. Louis, Missouri. 

d. Mr. Tompkin, a citizen and resident of Greece, is a partner in Sophic Partnership, which conducts business in 12 countries, including the United States.