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.