name写成 "arr[]"的形式就可以了

     
     
  
      
           本次作业标题      
             
       
    
       
               添加项目      
              音频文件      
              答案      
            
       
  
      
  
      
     
  添加输入项

button不能放在form里面，不然就会提交，不知道为什么

SO 里问了一下，回答是

The <button> element is a submit button by default. You can change this with the type="button"attribute, which makes it do nothing by default, or calling preventDefault on the event. But I'd go with the attribute since then your intention is semantically clear without actually running the script.