A. is influencing the racial barriers in the United States of America B. is softening the historical isolation of European ethnic immigrants C. is undergoing subtle shifts D. is now at work 参考答案:A 答案解析:文章最后两句指出异族通婚的上升趋势正在使种族的社会意义经历细微的变化,种族之间的壁垒正在被渗透。