
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
Determine the coefficient of $x^{3}$ in the expansion of $(1+x)^{3}+(1+x)^{4}+...+(1+x)^{2007}$.

A) $C_{2007}^{4}$
B) $C_{2007}^{3}$
C) $C_{2008}^{4}$
D) $C_{2008}^{3}$

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