Pandas DataFrame Copy

What happens when you modify df2 in this code?

df1 = pd.DataFrame({'A': [1, 2, 3]})
df2 = df1
df2['A'] = [4, 5, 6]
A

A copy-on-write warning is raised

B

Only df2 is modified

C

Both df1 and df2 are modified

D

Neither is modified