Sentence Correction

By the end of the nineteenth century, five of the Western European states had
developed a railroad system, but only one in the East.
(A) only one in the East
(B) only one eastern state
(C) in the East there was only one state
(D) in the East only one state did
(E) only one in the East had

doubt: i feel that D is a better answer as "only" is used in much better way in D than in E. what is so wrong in D?

Since we're comparing how many railroad systems had been developed, we need a verb in both parts of the comparison. D lacks an action verb to match "had been developed", so it's out.

5 European states had developed.....but
only 1 in the East had(developed...)
Two scenarios within the same timeframe.

C uses "did" to imply "did developed" a railway system??