如何将DataFrame的二级列索引合并成单个列索引？

import pandas as pd

data = {'A': [1, 2, 3], 'B': [4, 5, 6]}
columns = pd.MultiIndex.from_tuples([('C', 'X'), ('C', 'Y')])
df = pd.DataFrame(data, columns=columns)

df_stacked = df.stack()

df_merged = df_stacked.reset_index()

import pandas as pd

data = {'A': [1, 2, 3], 'B': [4, 5, 6]}
columns = pd.MultiIndex.from_tuples([('C', 'X'), ('C', 'Y')])
df = pd.DataFrame(data, columns=columns)

df_stacked = df.stack()
df_merged = df_stacked.reset_index()

print(df_merged)

   level_0 level_1  C
0        0       X  1
1        0       Y  4
2        1       X  2
3        1       Y  5
4        2       X  3
5        2       Y  6