The McMillan Glass Factory in the 19th century Europe was different from all other glass factories in that it required its workers to have taken a systematic course in glass-making or passing a special examination on glass-making skills; none of McMillan's workers were untrained.
A. to have taken a systematic course in glass-making or passing a special examination on glass-making skills; none of McMillan's workers were
B. to have taken a systematic course in glass-making or to had passed a special examination on glass-making skills and none of McMillan's workers was
C. either to have taken a systematic course in glass-making or to have passed a special examination on glass-making skills, with no worker of McMillan being
D. either to have taken a systematic course in glass-making or to have passed a special examination on glass-making skills; none of McMillan's workers was
E. either to have taken a systematic course in glass-making or to have passed a special examination on glass-making skills, resulting in none of McMillan's workers being
None of the workers takes a singular verb so were trained is wrong. parallelism between either to have taken or to have passed. when two independent clauses are joined without a subordinating or a coordinating conjunction then we can separate the two clauses by a semi-colon. So the ans is D