Re-word "no answer given" from passive to active.

lang/en/qtype_stack.php

@@ -398,7 +398,7 @@ $string['singlechargotmorethanone'] = 'You can only enter a single character her
 
 $string['true'] = 'True';
 $string['false'] = 'False';
-$string['notanswered'] = '(No answer given)';
+$string['notanswered'] = '(Clear all choices)';
 $string['ddl_runtime'] = 'The input has generated the following runtime error which prevents you from answering. Please contact your teacher.';
 $string['ddl_empty'] = 'No choices were provided for this drop-down.';
 $string['ddl_nocorrectanswersupplied'] = 'The teacher did not indicate at least one correct answer. ';