从两个类别列熊猫中创建一个类别列

I have this dataframe

我有这个dataframe

d = {
    'Primary area': [
        'Biological Sciences A',
        'Cultures and Cultural Production',
        'Mathematics'
    ],
    'Discipline': [
        'Biochemistry and Molecular Biology',
        'Philosophy',
        'Pure Mathematics'
    ]
}

import pandas as pd

df = pd.DataFrame(data=d)

    Discipline                              Primary area
0   Biochemistry and Molecular Biology      Biological Sciences A
1   Philosophy                              Cultures and Cultural Production
2   Pure Mathematics                        Mathematics
d = {