function change_class2(_b) {
	document.getElementById("m_c_n1").className="menu_desc";
	document.getElementById("m_c_n2").className="menu_desc";
	document.getElementById("m_c_n3").className="menu_desc";
	document.getElementById("m_c_n4").className="menu_desc";
var btn = document.getElementById(_b);
btn.className= "active_menu_desc";
}