Sentence Correction

Some soldiers that were killed and seriously injured in the fighting last year were not trained according to regulations.

A. Some soldiers that were killed and seriously injured in the fighting last year were not

B. In the fighting last year some of the soldiers that were killed or seriously injured were not

C. Last year some of the soldiers that were killed and seriously injured in last year's fighting had not been

D. Some soldiers that were killed or seriously injured in the fighting last year had not been

E. Some soldiers that were killed or seriously injured in last year's fighting have not been

OA is D
Please, I would need the help of an expert to explain the parallelism here. Thanks