More adeptly handling responsibility than his predecessors were able to do, the new owner of the property rapidly increased the production rate of the land.

A.More adeptly handling responsibility than his predecessors were able to do
B.As he was more adept at handling responsibility than were his predecessors
C.Handling more responsibilities adeptly than his predecessors
D.More adept than his predecessors at handling responsibility
E.Since he handled responsibility in a manner more adept than that of his predecessors

Question is from Kaplan 800. Ans is D. But I am confused between B and D. In D if we apply the idiom More X than Y how is adept and his predecessor parallel. I thought it should have been More adept than his predecessors were. Also I am always confused when then is used in the sentence can somebody throw some light on how to deal with then. Thank you