What will be the output of the following PHP code? <?php $a1 = array("red", "green"); $a2 = array("blue", "yellow"); $a3 = array_merge($a1, $a2); $a4 = array("a", "b", "c", "d"); $a = array_combine($a4, $a3); print_r($a); ?>

• A. Array ( [a] =&gt; blue [b] =&gt; yellow [c] =&gt; red [d] =&gt; green )
• B. Array ( [0] =&gt; blue [1] =&gt; yellow [2] =&gt; red [3] =&gt; green )
• C. Array ( [0] =&gt; red [1] =&gt; green [2] =&gt; blue [3] =&gt; yellow )
• D. Array ( [a] =&gt; red [b] =&gt; green [c] =&gt; blue [d] =&gt; yellow )