score:1

It looks like you are not returning the function that takes t as an argument. You can see the original tween has one more return than your changed version has.

return function(d, i) {
        return (i == 0) ?
            function(t) {
                a.domain(xd(t));
                b.domain(yd(t));
                return arc(d);
            } 
        : function(t) {
                return arc(d);
            };
    }

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