Which of the following transactions would contribute to a U.S. current account surplus?

a. Boeing barters a \($100\) million plane to Mexico in exchange for \($100\) million worth of hotel services on the Mexican coast.

b. The United States borrows \($100\) million long-term from Saudi Arabia to buy

\($100\) million of Saudi oil this year.

c. The United States sells a \($100\) million jet to Turkey, and Turkey pays by transferring the \($100\) million from its bank account to the U.S. seller.

d. A British investor buys \($100\) million of IBM bonds from the previous U.S. owner of these bonds, and the British buyer pays by transferring the \($100\) million from his bank account to the previous U.S. owner.