title: Names of the columns we're searching for missing values
body: <p>Searching for missing values ?</p>
  
  <pre><code>columns = ['median', 'p25th', 'p75th']
  
  # Look at the dtypes of the columns
  print(____)
  
  # Find how missing values are represented (Search for missing values in the median, p25th, and p75th columns.)
  print(recent_grads["median"].____)
  
  # Replace missing values with NaN,using numpy's np.nan.
  for column in ___:
      recent_grads.loc[____ == '____', column] = ____?
  </code></pre>