JQuery案例一：实现表格隔行换色

<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title>使用JQ完成表格隔行换色</title>
		<script src="js/jquery.min.js"></script>
		<script>
			$(function() {
				$("tbody tr:odd").css("background-color","aquamarine");
				$("tbody tr:even").css("background-color","bisque");
				var tb = $("tbody tr");
				var oldColor;
				for(var i=0;i<tb.length;i++) {
					//alert(oldColor);
					$("tbody tr")[i].onmouseover = function() {
						oldColor = this.style.backgroundColor;
						this.style.backgroundColor = "yellow";
					}
					$("tbody tr")[i].onmouseout = function() {
						this.style.backgroundColor = oldColor;
					}
				}
			});
		</script>
	</head>
	<body>
		<table id="tbl" border="1" border-collapse="collapse" align="center" cellspacing="0" cellpadding="5" width="400" height="20">
			<thead>
				<tr>
					<th>编号</th><th>姓名</th><th>年龄</th>
				</tr>
			</thead>
			<tbody>
				<tr>
					<td>1</td><td>张三</td><td>12</td>
				</tr>
				<tr>
					<td>2</td><td>李四</td><td>22</td>
				</tr>
				<tr>
					<td>3</td><td>王五</td><td>13</td>
				</tr>
				<tr>
					<td>4</td><td>马六</td><td>14</td>
				</tr>
				<tr>
					<td>5</td><td>伍六七</td><td>17</td>
				</tr>
				<tr>
					<td>6</td><td>梅花十三</td><td>17</td>
				</tr>
			</tbody>
		</table>
	</body>
</html>

疑问：为什么this.style.backgroundColor = oldColor中的this不能用$("tbody tr")[i]代替？？