WHY

@echo off & setlocal enabledelayedexpansion
for /f "delims=" %%i in (a.txt) do set /a n+=1 & set "_!n!=%%i"
set /a n = %Random% %% n + 1
set "str=!_%n%!" & echo;!str!
pause
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WHY

@echo off & setlocal enabledelayedexpansion
for /f %%i in ('find /v /c "" ^< a.txt') do set /a n = !Random! %% %%i + 1
(for /l %%i in (1 1 %n%) do set /p str=)<a.txt
echo;!str!
pause
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