Which of the following are not precautions for the preliminary test?

Which of the following are not precautions for the preliminary test?
A . Before the test, check whether the required test lines are complete, the type of connector and the applicability of the instrument, and whether the connection is good.
B. Prepare cotton swabs and alcohol, and pay attention to the cleanliness of the connectors at all times during the optical interface characteristic test to ensure the accuracy of the test data.
C. Be careful not to look directly at the optical sending interface during the test, so as not to damage the eyes.
D. During the 24-hour bit error test, it should be ensured that the entire test process is free from external interference (including human factors and power conditions, etc.). If there is an exception, the exact time of the exception should be recorded or re-tested. The test time does not need to exceed 24 hours.

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Answer: D