A D3.js visualization demonstrates how a circle's radius changes interactively. A slider input controls the circle's size, with the current radius value displayed as text. The page renders a single SVG circle centered on a 600 by 400 canvas. The code uses the `d3.select` API to bind the range input's `input` event to an `update` function, which modifies both the circle's `r` attribute and the displayed numeric label. The initial radius is set to 100, and the slider allows adjustments from 1 to 150.