'Grades the problem and returns its hint from cmap or the empty string.'

def get_hint(self, problem_id, choice):

    student_answers = {problem_id: choice}
    cmap = self.problem.grade_answers(answers=student_answers)
    adict = cmap.cmap.get(problem_id)
    if adict:
        return adict['msg']
    else:
        return ''
