O código PHP que imprime a mensagem "abcd" é:...

$x = array(?a? => ?b?, ?c? => ?d?); foreach($x as $y){ echo $y; }

$x = array(?a? => ?b?, ?c? => ?d?); foreach($x as $key => $value){ echo $key; }

$x = array(?a? => ?b?, ?c? => ?d?); foreach($x as $key => $value){ echo $value; }

$x = array(?a? => ?b?, ?c? => ?d?); foreach($x as $key => $value){ echo $key.$value; }

$x = array(?a? => ?b?, ?c? => ?d?); foreach($x as $key => $value){ echo $x; }