How can you add a background color for all <h1> elements using external CSS?
<h1>
<h1 style="background-color: blue;"> in the HTML file
<h1 style="background-color: blue;">
<link> h1 { background-color: blue; } </link> in the HTML file
<link> h1 { background-color: blue; } </link>
h1 { background-color: blue; } in the linked CSS file
h1 { background-color: blue; }
<style> h1 { background-color: blue; } </style> in the HTML file
<style> h1 { background-color: blue; } </style>