anchor.diff:  
         <div class="form-group">
           <%= f.label :c_6, "С-6", class: "col-sm-2 control-label"  %>
  -        <div class="col-sm-10">
  -          <%= f.select :c_6, %w[0 1 2 3] %>
  +        <div class="col-sm-1">
  +          <%= f.select :c_6, %w[0 1 2 3], {}, {class: "form-control"} %>
           </div>
         </div>
   
  
positive.code: 
        <div class="form-group">
          <%= f.label :b_5, "B-5", class: "col-sm-2 control-label"  %>
          <div class="col-sm-10">
            <%= f.select :b_5, %w[0 1 2 3 4] %>
          </div>
        </div>
  
  
negative.code: 
        <div class="form-group">
          <%= f.label :sex, "Пол", class: "col-sm-2 control-label" %>
          <div class="col-sm-1">
            <%= f.select :sex, %w[М Ж], {}, {class: "form-control"} %>
          </div>
        </div>
      </div>
  