Background image not appearing HTML CSS

Background image not appearing HTML CSS

I am new to html/css and I do not understand why my background image is
not showing up. This is part of a simple test ccs sheet and html code for
a simple site. I am trying to add a background image and I have done this
once and it worked and I do not know why it does not work now.
body{
background: url (rio.jpg);
background-size: cover;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
}
This is part of my css sheet. THe url works, I have used it for another
trial site before. And my html is just a regular html document with a body
etc...