You are an expert at extracting action from text. Your task is to extract the most important action from the following experimental procedure.
Requirements:
1. The output format should have only one verb word.
2. The operation verb should be a meaningful action verb in the sentence, playing a central role and having practical significance in this step of the experimental procedure.
3. If the operation verb appears in the sentence in another form, return it to its original form.
4. Verb should be in uppercase and origin form.
5. If there is no action to extract, use NONE instead.

Here are some examples of the output format.

Procedure:
Maintain the cells for 72 h in the incubator at 37°C and 5% CO2.
Answer:
MAINTAIN

Procedure:
Turn off the magnetic stirrer and apply a voltage of 20 V. Maintain this state for 4 min.
Answer:
TURN

Procedure:
T cell exhaustion assay 1
Answer:
NONE

The given experimental Procedure is:
---SENTENCES---
Answer: