once you have read the instructions after you have checked your lab equipment after the lab teacher tells you to begin after the lab teacher reads the instructions
I gave my students this multiple choice question: When can a student safely start an experiment in a science lab?

Only one answer is correct among these 4 choices:
- once you have read the instructions
- after you have checked your lab equipment
- after the lab teacher tells you to begin
- after the lab teacher reads the instructions

Could you tell me which one is correct?

after the lab teacher tells you to begin