
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
In a middle school, there are 36 volunteers learning about laws in the first grade, 72 in the second grade, and 54 in the third grade. Stratified sampling is used to select a sample from these three grades, and it is known that there are 3 volunteers from the third grade in the sample.
(I) Calculate the number of volunteers from the first and second grades in the sample;
(II) Let $A_i$ (i=1, 2...) represent the volunteers from the first grade in the sample, and $a_i$ (i=1, 2...) represent the volunteers from the second grade in the sample. If 2 people are randomly selected from all the volunteers in the first and second grades of the sample, ① list all possible pairs of volunteers using notation for volunteers mentioned above, ② calculate the probability that both selected volunteers are from the same grade.

Present the answer in LaTex format: \boxed{Your answer}
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