My sister has worked as an engineer since she _____ back from the United States.[ ]

A. came

B. had come

C. comes

D. has come

某数据库中有表COURSE(CNO，CNAME，TNO)和表TEACHER(TNO，TNAME，DEPART)，完成查询选修某课程的同学人数多于5人的教师姓名。
SELECT TNAME
FROM TEACHER WHERE
______
(SELECT X.TNO FROM COURSE X，COURSE Y
WHERE ______
______
______)