title: Difference between ?selectableItemBackground, ?android:selectableItemBackground and ?android:attr/selectableItemBackground in Android?
body: <p>What is the difference between:</p>
  
  <pre><code>android:background="?selectableItemBackground"
  
  android:background="?attr/selectableItemBackground"
  
  android:background="?android:selectableItemBackground"
  
  android:background="?android:attr/selectableItemBackground" 
  </code></pre>
  
  <p>in Android?</p>