Delete rows where all columns are NULL/NaN

I have a dataframe and want to delete the row where all columns are NULL.

Sno.        A           B           C          D 
  1          10           a                      4.3
  2
  3          14           b           n9         12
  4
  5                         c           w2        2.5
  6          57           d           h5         80

And I want

Sno.        A           B           C          D 
  1          10           a                      4.3
  2          14           b           n9         12
  3                         c           w2        2.5
  4          57           d           h5         80

How do I do that?

I would start with dropna