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from __future__ import print_function
import yt
import numpy as np

1. Load the `"virgo_novisc.0054.gdf"` dataset from the `"data"` directory.


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2. Create a sphere object centered on the domain center. Use the shorthand `"c"` for the center. Give it a radius of 200 kpc.


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3. Create a second sphere object at the location ``[0.1, -0.2, 0.3]``. Give it a radius of 0.4 Mpc.


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4. Query the sphere object for the density and calculate the mean using `np.mean`.


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