Boris is an unmarried systems specialist with a public accounting firm. During all of 2021, he is on temporary assignment in London. He pays $21,000 in British income tax on his $100,000 salary. Boris knows little about taxes and seeks your advice on the taxability of the salary he earns while in London. Write Boris a memorandum explaining the tax treatment of his London salary. Assume that Boris has no other income sources and that he does not itemize deductions.