title: How to filter missing data of spcific columns in pandas
body: <p>I have data like:</p>
  
  <pre><code>     a              b        c
  
  0  20120112        NaT    20120112
  
  1  NaT             NaT    20120112
  
  2  20120112        NaT    NaT    
  </code></pre>
  
  <p>I want to filter out row "NaT NaT 20120112", because column a and column b are both NaT in that row. How can I achieve that?</p>