
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 following four propositions:
① From a uniformly moving production line, a quality inspector takes one product every 10 minutes for a certain index test. Such sampling is stratified sampling.
② The stronger the linear correlation between two random variables, the closer the absolute value of the correlation coefficient is to 1.
③ For a measurement result $\xi$ that follows the normal distribution $N(1, a^2)$, if $P(\xi \leq 5) = 0.81$, then $P(\xi \leq -3) = 0.19$.
④ For the observed value $k$ of the random variable $K^2$ of two categorical variables $X$ and $Y$, the smaller $k$ is, the more confident we are in determining that "X is related to Y".
The number of true propositions among the above is.

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