Write code that finds all lines in the Pandas dataframe without the "%" sign and replace all their content with NaN

import pandas as pd
df = pd.read_csv('studentscores.csv')

def clean_data(df):
  for index, row in df.iterrows():
    for i, item in enumerate(row):
      if type(item) == str:
        if '%' in item:
          df.iloc[index, i] = item.replace('%', '').replace(',', '')
        else:
          df.iloc[index, i] = float('NaN')
  return df

clean_data(df)