title: Which of the following are true for singleton classes?
body: <p>dear all sorry for asking basic question.. i still did not understand about singleton.
  i still can not figure the answer.</p>
  
  <p>Which of the following are true for singleton classes?
  a. Cannot be derived by other classes.
  b. Cannot be referred to in a variable.
  c. Not more than one instance may exist ever within a runtime.
  d. They are thread safes</p>