[
   {
    "pid": 1,
    "question":"这是什么类型的水工挡土墙? ",
    "image":"images/1.jpg",
    "choices": ["重力式","衡重式","悬臂式","板桩式","锚杆式","加筋式","扶壁式"],
    "unit":"null",
    "precision":"null",
    "answer":"重力式",
    "question_type": "mlti_choice",
    "answer_type": "text"
  },
  {
   "pid": 2,
   "question":"水工挡土墙的顶标高是多少？",
   "image":"images/2.jpg",
   "choices": "null",
   "unit": "m",
   "precision": "2",
   "answer": "302.70",
   "question_type":"free_form",
   "answer_type":"float"
  },
  {
   "pid": 3,
   "question":"水工挡土墙的底标高是多少？",
   "image":"images/2.jpg",
   "choices": "null",
   "unit": "m",
   "precision":  "2",
   "answer": "296.00",
   "question_type":"free_form",
   "answer_type":"float"
   }
]