In the accompanying figure, segments AB and CD are parallel, the measure of angle D is twice the measure of angle B, and the measures of segments AB and CD are a and b respectively. Then the measure of AB is equal to<spanclass=′latex−bold′>(A)</span>21a+2b<spanclass=′latex−bold′>(B)</span>23b+43a<spanclass=′latex−bold′>(C)</span>2a−b<spanclass=′latex−bold′>(D)</span>4b−21a<spanclass=′latex−bold′>(E)</span>a+b[asy]
size(175);
defaultpen(linewidth(0.8));
real r=50, a=4,b=2.5,c=6.25;
pair A=origin,B=c*dir(r),D=(a,0),C=shift(b*dir(r))*D;
draw(A--B--C--D--cycle);
label("A",A,SW);
label("B",B,N);
label("C",C,E);
label("D",D,S);
label("a",D/2,N);
label("b",(C+D)/2,NW);
//Credit to djmathman for the diagram[/asy]