Which of the following code segments can be used to check the form validity?

A.
if (Zend_Form::isValid($_Post)) {
// success!
} else {
// failure!
}

B.
if (Zend_Form->isValid($form)) {
// success!
} else {
// failure!
}

C.
if ($form->isValid($_POST)) {
// success!
} else {
// failure!
}

D.
if (Zend_Form::isValid($form)) {
// success!
} else {
// failure!
}

Explanation: