Which Two of the Following Will Return an Instance of a Block

Which two of the following will return an instance of a block object? (Choose two)

A) Mage::getBlock('page/html_header') ;
B) Mage::app() ->createBlock('page/html_header') ;
C) Mage::createBlock('page/html_header') ;
D) Mage::app() ->getLayout() ->createBlock('page/html_header') ;
E) Mage::getLayout() ->createBlock('page/html_header') ;
F) Mage::app() ->getLayout() ->createBlock('page/html_header') ->setTemplate('page/html/header=phtml') ;