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I complete the function as follows: import pandas as pd import numpy as np df1 = pd.DataFrame(np.random.randn(10,4), index=pd.date_range('2021-01-21',periods=10), columns=list("ABCD")) df2 = pd.DataFrame(np.random.randn(10,4), index=pd.date_range('2021-01-21',periods=10), columns=list("ABCD")) co...