This example demonstrates a minimal interactive D3.js visualization: a single circle rendered as an SVG element. The circle changes its fill color and radius when hovered, using a CSS transition for a smooth three-second animation. The code relies on D3 version 7, loaded from a CDN, and manipulates the SVG circle's attributes directly through CSS and HTML, without invoking any specific D3 functions or data binding. The visualization serves as a basic starter template for D3 projects.