
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 the set of integers $Z$, all integers that have a remainder of $k$ when divided by 5 form a "class", denoted as $[k]$, i.e., $[k]=\{5n+k|n\in Z\}$, where $k=0, 1, 2, 3, 4$. Given the following four statements:  
① $2011 \in [1]$;  
② $-3 \in [3]$;  
③ $Z=[0] \cup [1] \cup [2] \cup [3] \cup [4]$;  
④ "The necessary and sufficient condition for integers $a, b$ to belong to the same 'class'" is "$a-b \in [0]$".  
Among these, the correct statements are.

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