
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
Spring has arrived, and the lotus leaves in a pond are thriving. It is known that the area of the pond's surface covered by new lotus leaves each day is twice that of the previous day. If it takes 20 days for the lotus leaves to completely cover the pond's surface, then when the lotus leaves just cover half of the pond's surface, the lotus leaves have been growing for ( )  
a. 10 days b. 15 days    c. 19 days    d. 20 days

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