Quiz — merge, + and explode

<?php

$a = [1, 2];
$b = [3];
$c = ['x' => 1];
$d = ['x' => 2, 'y' => 3];
echo json_encode(array_merge($a, $b)), ' ', json_encode($a + $b), ' ';
echo json_encode($c + $d), ' ', json_encode([...$a, ...$b]), "\n";