为什么tr:悬停不工作

I am a bit new to CSS and am stuck with this small problem. I want the background color of a table row to change when I hover over that row. I have written the code below but somehow only the hovering part does not seem to work. I have tried viewing in Google Chrome v24 and Firefox v18. Could anyone say where am I going wrong. Thanks.I am a bit new to CSS and am stuck with this sm