Python: extracting column from np array and making it flat

>>> import numpy as np
>>> A = np.array([[1,2,3,4],[5,6,7,8]])

>>> A
array([[1, 2, 3, 4],
    [5, 6, 7, 8]])

>>> A[:,2] # returns the third columm

>>> import pandas as pd

>>>aa = pd.DataFrame(A[:,2].ravel())

>>>aa