A line of twenty gold circles with black outlines moves vertically down the viewport, then reverses direction and moves up, bouncing between the top and bottom edges. The animation updates every 200 milliseconds using `setInterval`, with each frame positioning circles via `d3.selectAll('circle').data().join()`. The `cx` and `cy` attributes are computed from the circle radius and an incrementing time variable, creating a simple looping motion. Rendered as SVG with D3 v7, the example fills the browser window.