
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
Let the universal set be $U$. Among the following conditions, the necessary and sufficient condition for $B \subseteq A$ is ____. (Fill in the correct proposition number in the blank)

1. $A \cup B = B$
2. $C_U B \subseteq C_U A$
3. $A \cup (C_U B) = U$
4. $A \cap (C_U B) = \emptyset$

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