What is the output of the following code?
function ratio ($xl = 10, $x2)
{
}
if (isset ($x2)) {
return $x2 / $xl;
echo ratio (0);
A . 0
B . An integer overflow error
C . A warning, because $xl is not set
D . A warning, because $x2 is not set
E . A floating-point overflow error
F . Nothing
Answer: D