What was the principle reason that the Kapteyn model and the Shapley model of the size of the Galaxy were incorrect?

W) They only examined the distribution of stars and failed to examine the distribution of globular clusters

X) They used data that was not sensitive enough to detect the faintest stars in the Galaxy

Y) They failed to account for extinction due to dust

Z) They only examined the distribution of globular clusters and failed to examine the distribution of stars
Y) THEY FAILED TO ACCOUNT FOR EXTINCTION DUE TO DUST