What is the output of the following script?
<code>
1 <?php
2 function ratio ($x1 = 10, $x2)
3 {
4 if (isset ($x2)) {
5 return $x2 / $x1;
6 }
7 }
8
9 echo ratio (0);
10 ?>
</code>

A.
0

B.
An integeroverflow error

C.
A warning, because $x1 is not set

D.
A warning, because $x2 is not set

E.
A floating-point overflow error