teacher:Good afternoon,everyone. This experiment is to start at 3 o’clock. Have you gone to the instructions?

student:Yes,Sir.

teacher:Ok. Now,some points for attention. First of all,pay attention to safety. Now,sign your name on the lab record. And after that you can start. Is there any question just let me know.

student:Excuse me,Sir. This air compressor doesn’t work.

teacher:Have you turned it on?

student:Yes,I have.

teacher:Ok,I’ll give you another one.

student:Thank you.