Collect values of pandas dataframe column A if column B is NaN (Python)

I have a pandas dataframe.

I want to collect/print the values of column A where column B is NaN.

Question How do I do this?

Edit Further: Say I have a set of columns (b,c,d). I want to select the values of column a if either b,c, or d is NaN.

(The trick for identifying NaNs is a bit different than simply "==" etc.)

Thank you