Which of the following will NOT display the value of $debug_variable?

ob_start();

Zend_Debug::dump($debug_variable, 'debug_variable', false);

ob_end_flush();

print(Zend_Debug::dump($debug_variable, 'debug_variable', true));

echo Zend_Debug::dump($debug_variable, 'debug_variable', false);

Zend_Debug::dump($debug_variable, 'debug_variable', true);