function cent(id){
		if(id==1){
		document.getElementById('cent_1').className="bk02";
		document.getElementById('cent_2').className="bk01";
		document.getElementById('cent_3').className="bk01";
		document.getElementById('cent_4').className="bk01";
		document.getElementById('cent_1c').style.display="block";
		document.getElementById('cent_2c').style.display="none";
		document.getElementById('cent_3c').style.display="none";
		document.getElementById('cent_4c').style.display="none";
		}
		else if(id==2){
		document.getElementById('cent_1').className="bk01";
		document.getElementById('cent_2').className="bk02";
		document.getElementById('cent_3').className="bk01";
		document.getElementById('cent_4').className="bk01";
		document.getElementById('cent_1c').style.display="none";
		document.getElementById('cent_2c').style.display="block";
		document.getElementById('cent_3c').style.display="none";
		document.getElementById('cent_4c').style.display="none";
		}
		else if(id==3){
		document.getElementById('cent_1').className="bk01";
		document.getElementById('cent_2').className="bk01";
		document.getElementById('cent_3').className="bk02";
		document.getElementById('cent_4').className="bk01";
		document.getElementById('cent_1c').style.display="none";
		document.getElementById('cent_2c').style.display="none";
		document.getElementById('cent_3c').style.display="block";
		document.getElementById('cent_4c').style.display="none";
		}

		else if(id==4){
		document.getElementById('cent_1').className="bk01";
		document.getElementById('cent_2').className="bk01";
		document.getElementById('cent_3').className="bk01";
		document.getElementById('cent_4').className="bk02";
		document.getElementById('cent_1c').style.display="none";
		document.getElementById('cent_2c').style.display="none";
		document.getElementById('cent_3c').style.display="none";
		document.getElementById('cent_4c').style.display="block";
		}
		}