写出命题“若x2+y2=0，则xy=0”的逆命题、否命题、逆否命题，并判断其真假．
解：命题“若x2+y2=0，则xy=0”的逆命题是“若xy=0，则x2+y2=0”，是假命题；
否命题是“若x2+y2≠0，则xy≠0”，是假命题；
逆否命题是“若xy≠0，则x2+y2≠0”，是真命题．
The fact that Mary was foreign made ________difficult for her to get a job in that country.
