Make radio subexercises of multiple problems work.

First, radio choices are defined as sibling to the solution element;
radio setup() uses that list to make a series of inputs with labels.
Multiple setup() copies everything in the solution (usually just a
couple of .sol subexercises) to the solution area. The choices list
needs to be removed not to appear "repeated" on the problem page.

Second, radio precalculates the index of the correct choice in the
setup() and passes this as additional info to its createValidator().
However, multiple passes just the respective .sol element to any of
its subproblem createValidator() functions. For now, radio-in-multiple
falls back to old radio code that does not need the additional info.

Some more things to do (deserving a separate branch):
* verify that choices are always defined just after the radio and
  turn .siblings('.choices') in radio setup() into .next('.choices'),
  so that a multiple can work with more than one radio subproblem;
* move the correct-index logic from radio setup() to createValidator();
* update the note on radios under Answer Types / multiple in the wiki.

test/answer-types.js

@@ -841,6 +841,31 @@
         start();
     });
 
+    asyncTest("multiple with radio (category)", 12, function() {
+        setupSolutionArea();
+        var $problem = jQuery("#qunit-fixture .problem").append(
+            "<div class='solution' data-type='multiple'>" +
+                "<div class='sol' data-type='radio'>5<\/div>" +
+                "<ul class='choices' data-category='true'>" +
+                    "<li>6<\/li>" +
+                    "<li>5<\/li>" +
+                    "<li>7<\/li>" +
+                "<\/ul>" +
+            "<\/div>"
+        );
+
+        var answerData = Khan.answerTypes.multiple.setup($("#solutionarea"),
+                $problem.children(".solution"));
+
+        testMultipleAnswer(answerData, [false, true, false], "right", "right answer is right");
+        testMultipleAnswer(answerData, [false, false, false], "empty", "empty answer is empty");
+        testMultipleAnswer(answerData, [false, false, true], "wrong", "wrong answer is wrong");
+        testMultipleAnswer(answerData, [true, false, false], "wrong", "wrong answer is wrong");
+
+        start();
+    });
+
+
     asyncTest("set with no things", 15, function() {
         setupSolutionArea();
         var $problem = jQuery("#qunit-fixture .problem").append(

utils/answer-types.js

@@ -1114,6 +1114,9 @@ Khan.answerTypes = $.extend(Khan.answerTypes, {
                 // solutionarea
                 var solarea = $(this).empty();
 
+                // If it is a radio it also has choices as a sibling element.
+                $(this).next(".choices").remove();
+
                 // perform setup on each of the areas
                 var answerData = Khan.answerTypes[type].setup(solarea, sol);
 
@@ -1675,6 +1678,10 @@ Khan.answerTypes = $.extend(Khan.answerTypes, {
         createValidator: function(solution) {
             // TODO(emily): Remove this backwards compatible code sometime
             // after 8/2013
+            // TODO(wrwrwr): Multiple setup calls createValidator() with .sol
+            // elements of subproblems. Therefore, index and noneIsCorrect will
+            // be missing (index will actually be a jQuery-defined function).
+            // This should be resolved before removing the compatibility code.
             var correct = extractRawCode(solution.solution || solution);
 
             function showReal() {
@@ -1706,7 +1713,7 @@ Khan.answerTypes = $.extend(Khan.answerTypes, {
                     return score;
                 }
 
-                if (guess.index) {
+                if (guess.index && solution.noneIsCorrect !== undefined) {
                     // New solutions include information about the correct
                     // answer like the correct index, etc. We can use that to
                     // make checking a lot simpler.