Write the function selectCircles to return a d3 selection of all the
circles that are children of the group-2 element.
<body>
<div>
<svg id="svg">
<g id="group-1">
<circle cx="100" cy="50" r="20" fill="red"></circle>
<circle cx="130" cy="50" r="20" fill="green"></circle>
<circle cx="160" cy="50" r="20" fill="blue"></circle>
</g>
<g id="group-2">
<circle cx="100" cy="90" r="20" fill="cyan"></circle>
<circle cx="130" cy="90" r="20" fill="magenta"></circle>
<circle cx="160" cy="90" r="20" fill="yellow"></circle>
</g>
</svg>
</div>
<script>
var sel = selectCircles(); // write this function
</script>
</body>