
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
There is a stone arch bridge in the shape of a parabola. At normal water level, the width of the water under the bridge is 20 meters, and the highest point of the arch is 4 meters above the water surface.
1. Based on the normal water level, when the water level rises by \( h \) meters, the width of the water under the bridge is \( d \) meters. Find a function that expresses \( d \) in terms of \( h \).
2. Assuming the water depth under the bridge at normal water level is 2 meters, to ensure that passing ships can navigate smoothly, the width of the water under the bridge must be no less than 18 meters. When will navigation be affected?

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