input: Write a function to filter columns that start with 'A' from a DataFrame.
  assert filter_A_columns(pd.DataFrame({'A1': [1, 2], 'B1': [3, 4], 'A2': [5, 6]})).columns.tolist() == ['A1', 'A2']
  assert filter_A_columns(pd.DataFrame({'A_col': [7, 8], 'B_col': [9, 10], 'C_col': [11, 12]})).columns.tolist() == ['A_col']
  assert filter_A_columns(pd.DataFrame({'B1': [13, 14], 'B2': [15, 16]})).columns.tolist() == []
output: def filter_A_columns(df):
      return df.loc[:, df.columns.str.startswith('A')]