
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
Among the three conditions: ① $x\in A$ is a sufficient but not necessary condition for $x\in B$; ② $A\cup B=B$; ③ $A\cap B=\varnothing$, choose one to fill in the blank in question (2) and solve the following problems:<br/>Given sets $A=\{x\left|\right.a-1\leqslant x\leqslant a+1\}$, $B=\{x\left|\right.-1\leqslant x\leqslant 4\}$.<br/>(1) When $a=2$, find $A\cup B$;<br/>(2) If we choose ____, find the range of real number $a$.

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