
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
$F(n)$ is a proposition about the natural number $n$. If $F(k)$ ($k \in \mathbb{N}^+$) is true, then $F(k+1)$ is true. It is known that $F(7)$ is not true, then we have: ① $F(8)$ is not true; ② $F(8)$ is true; ③ $F(6)$ is not true; ④ $F(6)$ is true; ⑤ $F(5)$ is not true; ⑥ $F(5)$ is true. Among these, the true propositions are $\boxed{\text{D. ③④}}$.

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