题目

—The final round of the competition is coming near. Can I really do well this time?—Believe in ______. You're always the best in our school. A . you B . your C . yours D . yourself 答案：D19．集合A={x|x是平面内的三角形}，B={x|x是平面内的矩形}，C={x|x是平面内的圆}，D={x|x＞0}，给出下列关系：①f：A→C，作三角形的内切圆；②f：C→B，作圆的内接矩形；③f：A→C，作三角形的外接圆；④f：C→A，作圆的内接三角形；⑤f：B→D，求矩形的对角线长；⑥f：C→D，求圆的周长；其中不是映射的序号为②④．