Why was the city of nanjing extremely vulnerable to japanese attack? ?

Thank you for posting your question here at . Below are the choices. I belive the answer is A. 

A) the city had been barricaded by the Japanese before the attack 

B) the Chinese armed forces had been removed from the city 

C) the food supply to the city had been cut off leaving the citizens in poor health 

D) the citizens were not allowed to keep any weaponry for their defense