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$arFilter = array('IBLOCK_ID' => $IBLOCK_ID, 'ACTIVE' => 'Y'); 
$arSelect = array('ID', 'NAME');
$rsSection = CIBlockSection::GetTreeList($arFilter, $arSelect); 
while($arSection = $rsSection->Fetch()) {
   echo ' - '.$arSection['NAME'].'<br>';
}