GMAT Sentence Correction (SC) | Re: SC:Conquistadors began the destruction of South American

My take on C vs E:
Conquistadors began the destruction of South American cities, which were characterized by ornate gold decorations, large populations. -- Here it says Conquistadors began destructing a cities which were characterized by ornate gold decorations, large populations. In fact, when the destruction started, the cities are still characterized by it. Use of 'were' makes it feel that it used to be characterized (event in past), but
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