function selectMenu(sectionId, mainMenuId, menuItemId) {
    if (document.getElementById(sectionId) != null){
        document.getElementById(sectionId).style.display = "block";
    }
    if (document.getElementById(mainMenuId) != null) {
        document.getElementById(mainMenuId).className = "mainSelected";
    }
    if (document.getElementById(menuItemId) != null) {
        document.getElementById(menuItemId).className = "subSelected";
    }
}