title: What can't be stored in NSDictionary?
body: <p>I have an online quiz I am trying to complete and this is the question: </p>
  
  <p>One of the following can't be stored in NSDictionary. It can't be stored because it is a _____ type:</p>
  
  <p>UIView
  NSNumber
  NSInteger
  UIColor</p>
  
  <p>Fill in in the blank. </p>
  
  <hr>
  
  <p>I can't seem to find the answer anywhere online. As far as I can tell they all have a way of being stored in NSDictionary, any ideas? </p>