Given the following dataframe:

d = {'one': pd.Series([10, 20, 30, 40],
                      index=['a', 'b', 'c', 'd']),
     'two': pd.Series([0, 0, 25, 0],
                      index=['a', 'b', 'c', 'd'])}
 
df = pd.DataFrame(d)


   one  two
a   10    0
b   20    0
c   30   25
d   40    0

How do I select the appropriate value based on the following if/else and populate it into a new column called df['three']?

df['three'] = If df['two']>0, select df['two'], else select df['one']

Basically, I am looking to have the final result as:

   one  two  three
a   10    0     10
b   20    0     20
c   30   25     25
d   40    0     40