Phil, who is from canada, became a naturalized u.s. citizen in october. in december, he married martina, who is an argentinean, in saratoga springs, new york. their tax advisor should inform them that, in order to file a joint return, at least one spouse must be a u.s. citizen or resident, and that, in their case, phil's citizenship would be determined: A. at end of the tax year. B. at beginning of the tax year. C. on the due date of the return. D. on april 15th of the calendar year