The uniformized set of characters, which some historians date in the late Qing dynasty, was the key to the sustainability and prosperity of the Chinese culture over thousands of years.

A. The uniformized set of characters, which some historians date
B. The uniformized set of characters, which some historians have thought to occur
C. Uniformizing the set of characters, dated by some historians at
D. The uniformization of a set of characters, thought by some historians to have occured
E. The set of characters' uniformization, dated by some historians to have been

Please explain your answer.