function change(obj){
 //obj.style.display = "none";
 
 if(obj.type=="text")  {  
    document.getElementById('password').style.display = "block";
    document.getElementById('password').focus();//加上
	obj.value='';
	document.getElementById('txt').style.display = "none";
	
    }else{
		obj.type='password';
        document.getElementById('txt').style.display = "none";
    }
 }