Plugging checkboxes into answer key

I'm working on a multi-question quiz powered by PHP and jQuery. After selecting some answers and clicking the Submit button, a user is forwarded to another page which has this answer key:

$totalCorrect = 0;
$answers = [1 => 'A', 2 => 'Jupiter', 3 => 'C', 4 => 'D', 5 => 'A', 6 => 'C', 7 => 'C', 8 => 'C', 9 => 'B', 10 => ''.$Ch.''];

foreach ($answers as $num => $answer)
{
 $question = 'q'.$num;
 if (isset($_POST[$question]) && $_POST[$question] === $answer)
 {
   $totalCorrect++;
 }
}

$pct = round( (($totalCorrect/count($answers)) * 100), 0);
echo $totalCorrect.' correct ('.$pct.'%)';

All are multiple-choice answers except for #2 (fill-in-the blank) and the last one, which asks users to check all the checkboxes that are correct. Everything works fine except for the last one.

All the code I posted above is preceded by this script, which is designed to make the checkbox answers a little more manageable:

if (isset($_POST))
 {
    // Check that the 2 correct answers are set, and the 3 incorrect  answers are not set
    if (isset($_POST['q10-A'], $_POST['q10-B'], $_POST['q10-C']) &&
        !isset($_POST['q10-D']) && 
        !isset($_POST['q10-E']) && 
        !isset($_POST['q10-F'])) 
    {
        $Ch = 'A';
    }

    else
    {
    $Ch = 'B';
    }

}

As you can see, I tried to make it work by giving the value 'A' to a correct radio checkbox group selection (A, B and C, in this case). But it doesn't work. If I replace ''.$Ch.'' with 'A' it still doesn't work.

$answers = [1 => 'A', 2 => 'Jupiter', 3 => 'C', 4 => 'D', 5 => 'A', 6 => 'C', 7 => 'C', 8 => 'C', 9 => 'B', 10 => ''.$Ch.''];

So I'm confused. Can anyone either show me the correct way to do what I'm trying to do or show me an alternative?