
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 triangle ABC, let $\angle A = 60^\circ$, $\angle B = 70^\circ$. There is an external point D such that $\angle BDC$ is an exterior angle at point B. If $\angle DBC = 40^\circ$, calculate the measure of $\angle BDC$.
A) $90^\circ$
B) $100^\circ$
C) $110^\circ$
D) $120^\circ$

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