Bat如何判断txt文本中第8行是否只有"符号，有则删除整行

@echo off
::新建文件夹，用于存放修改后的文件
md NewFolder
setlocal enabledelayedexpansion
for %%i in (*.txt) do (
    set n=0
    (for /f "delims=" %%a in (%%i) do (
        set /a n+=1
        if !n! equ 8 (
            set str=%%a
            set "str=!str:"=!"
            if "!str!"=="%%a" echo,%%a
        ) else echo,%%a
    ))>NewFolder\%%i
)
pause