Build a line chart representing the level of CO2 emissions (metric tonnes of CO2 per person) for the US. Follow the next steps:

1. Import the matplotlib.pyplot under the plt alias.
2. Save the United States data in the usa variable.
3. Create Figure and Axes objects using the .subplots() method of plt and assign them to variables fig, and ax, respectively.
4. Initialize a line chart:
   • display years on the x-axis (use the .index attribute), convert them to integers (int);
   • display level of emissions on the y-axis (use the .values attribute).
5. Display the plot.

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