Trumpler Discovered the Effect of Interstellar Dust by Comparing Two

Trumpler discovered the effect of interstellar dust by comparing two different distance measurements of distant clusters.The first was based on appearance (concentration of stars toward the center,etc.) ,which was unaffected by dust.The other was spectroscopic parallax (use of the H-R diagram) ,which was affected by dust.What should such a comparison show?

A) The H-R diagram should always give larger distances.
B) The H-R diagram should always give smaller distances.
C) There should be no difference in the distances.
D) The distances should be different, but sometimes the H-R diagram should give the larger distance and sometimes the smaller depending on the actual distance to the cluster.