
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 a sequence $\{u_n\}$ be defined by $u_1=7$ and the recursive relationship $u_{n+1}-u_n=1+3n$ for $n=1,2,3,\cdots$. If $u_n$ is expressed as a polynomial in $n$, the algebraic sum of its coefficients is:
A) 6.5 
B) 7.5 
C) 7 
D) 6 
E) 8

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