This example demonstrates dynamic selection and updating of SVG circles using D3's modern `select` and `join` APIs. Fifteen circles animate horizontally across the window, with their vertical positions following a sine wave that shifts over time. The code uses `d3.select` to target the body, `d3.range` to generate data, and the `.join()` method to handle enter, update, and exit selections for circle elements, updating every frame via `setInterval`.