the teacher has given permission, allowed to use the equipment in the lab.
A. Only if, are the students
B. If only, the students are
C. If only, are the students
D. Only if the students are
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(答案→)A. Only if, are the students
B. If only, the students are
C. If only, are the students
D. Only if the students are
解析：句意：只要老师答应，学生被允许使用实验室的设备。这里是only修饰if的从句放在句首时，句子的主句用部分倒装，故选A。