create new column using if , else statments from lists

i need to create a new column C using if and else statements, from A, B columns: as in example

the below code returns nothing, can anybody notify me the correct one

import numpy as np
import pandas as pd
a = np.arange(10)
b = [0.1,0.3,0.1, 0.2, 0.5, 0.4,0.7,0.56, 
0.78, 0.45]
df= pd.DataFrame(data=b, columns=['B'])
df2= pd.DataFrame(data=a, columns=['A'])
A = df2['A']
B = df['B']
print (A, B)

def comma ( A, B, c):

  if B >= 0.1 and B <0.4:
    c = B *2
  else:
    c = B*A

 print (c)