实体框架多对多关系错误

我正在尝试在我的两个表之间创建多对多关系，但是当我运行 Update-Database 命令时，我收到此错误：

在表'ExamQuestions'上引入FOREIGN KEY约束'FK_dbo.ExamQuestions_dbo.Questions_Question_Id'可能会导致循环或多个级联路径 . 指定ON DELETE NO ACTION或ON UPDATE NO ACTION，或修改其他FOREIGN KEY约束 . 无法创建约束或索引 . 查看以前的错误 .

我的第一个实体是：

public class Question
{
   public Question()
   {
       this.Exams = new HashSet<Exam>();
   }

   [Key]
   public int Id { get; set; }

   [Required(ErrorMessage="Question is Required")]
   [Display(Name="Question")]
   [AllowHtml]
   public string QuestionText { get; set; }

   // public bool IsMultiSelect { get; set; }
   public string Hint { get; set; }
   public string HelpLink { get; set; }

   public int Marks { get; set; }
   public byte[] ImageData { get; set; }
   [StringLength(50)]
   public string MimeType { get; set; }
   public byte[] Audio { get; set; }

   public int QuestionTypeId { get; set; }
   public int TopicId { get; set; }
   public int DifficulityLevelId { get; set; }
   public int SubjectId { get; set; }

   public DifficultyLevel QuestionDifficulity { get; set; }
   public Topic Topic { get; set; }

   public virtual ICollection<Option> Options { get; set; }
   public ICollection<Exam> Exams { get; set; }
}

第二个实体是：

public class Exam
{
   public Exam()
   {
       this.Questions = new HashSet<Question>();
   }

   [Key]
   public int Id { get; set; }

   [Required]
   public string Name { get; set; }
   [Required]
   public int Duration { get; set; }
   [Required]
   public int TotalQuestion { get; set; }
   [Required]
   public int TotalMarks { get; set; }
   public bool SectionWiseTime { get; set; }
   public bool QuestionWiseTime { get; set; }
   public bool AllQustionRequired { get; set; }
   public bool AllowBackForward { get; set; }
   public bool SuffleSubjectWise { get; set; }
   public bool SuffleOptionWise { get; set; }
   public bool GroupSubjectWise { get; set; }

   [Required]
   public int ExamTypeId { get; set; }
   [Required]
   public int ExamInstructionId { get; set; }
   [Required]
   public int DifficultyLevelId { get; set; }

   public virtual ExamType ExamType { get; set; }
   public virtual ExamInstruction ExamInstruction { get; set; }
   public virtual DifficultyLevel DifficultyLevel { get; set; }

   public virtual  ICollection<Question> Questions { get; set; }
   //public virtual ICollection<ExamSchedule> ExamSchedules { get; set; }
}

谁能告诉我哪里出错了？