JavaScript中的if...else if...语句

1、源码
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>JavaScript</title>
<script type="text/javascript">
    var score = prompt("请输入一个分数","");
	if(!score){
		alert("您没有输入值！");
	}else{
		score *= 1;
		if(isNaN(score))
		{
			alert("您输入不合法的值！");
		}else{
			if(score >= 0 && score < 60)
			{
				alert("E");
			}else{
				if(score >= 60 && score < 70)
				{
					alert("D");
				}else{
					if(score >= 70 && score < 80)
					{
						alert("C");
					}else{
						if(score >= 80 && score < 90)
						{
							alert("B");
						}else{
							if(score >= 90 && score <= 100)
							{
								alert("A");
							}
						}
					}
				}
			}
			
		}
		
	}
</script>
</head>

<body>
</body>
</html>