private void lblClearQuestions_Click(object sender, EventArgs e)
 {
     if (false == QuestionManager.ClearErrorQuestion())
     {
         MessageBox.Show("清空题目失败", "错误提示", MessageBoxButtons.OK, MessageBoxIcon.Error);
     }
     else
     {
         MessageBox.Show("清空题目成功", "消息提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
     }
 }