A.SELECT姓名,单位FROM读者; WHERE借书证号IN; (SELECT借书证号FROM借阅; GROUP BY借书证号HAVING COUNT(*)>=2) B.SELECT姓名,单位FROM读者; WHERE借书证号EXISTS; (SELECT借书证号FROM借阅; GROUP BY借书证号HAVING COUNT(*)>=2) C.SELECT 姓名,单位 FROM 读者; WHERE 借书证号 EXISTS; (SELECT 借书证号 FROM 借阅; GROUP BY 借书证号 WHERE COUNT(*)>=2) D.SELECT 姓名,单位 FROM 读者; WHERE 借书证号 IN; (SELECT 借书证号 FROM 借阅; GROUP BY 借书证号 WHERE COUNT(*)>=2)