问题补充：
Only when permission allowed to enter the lab.
A. a teacher has given; are the students
B. has a teacher given; are the students.
C. has a teacher given; the students are
D. a teacher has given; the students are
答案：
(答案→)A. a teacher has given; are the students
B. has a teacher given; are the students.
C. has a teacher given; the students are
D. a teacher has given; the students are
解析：only 引导的状语从句放句首时，主句要倒装。
