score:2

This should do the trick:

svg.append("svg:circle")
    .attr("cx", 0)
    .attr("cy", 0)
    .attr("r", 20)
    .style("fill", "white")
    .append("g");

Alternatively a better approach is to do something like this:

svg.append("svg:circle")
    .attr("cx", 0)
    .attr("cy", 0)
    .attr("r", 20)
    .attr("class", "white-circle")
    .append("g");

Then, in CSS:

.white-circle {
    fill: #FFF;
}

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